diff --git a/.claude/rules/ruby/smalruby3.md b/.claude/rules/ruby/smalruby3.md
index f3e1990f75c..b6cc3a7308a 100644
--- a/.claude/rules/ruby/smalruby3.md
+++ b/.claude/rules/ruby/smalruby3.md
@@ -12,6 +12,50 @@ paths:
 
 smalruby3 は scratch-vm の Ruby 実装。smalruby3-editor で生成した Ruby スクリプトをデスクトップでネイティブ実行する。
 
+## RubyGems 公開
+
+- **gem 名**: `smalruby3`（https://rubygems.org/gems/smalruby3）
+- **所有者**: Kouji Takao
+
+### バージョニング: YY.MR.DDR
+
+semantic versioning 互換かつリリース日推測可能な形式:
+
+| フィールド | 意味 | 例 |
+|-----------|------|-----|
+| `YY` (MAJOR) | 年の下2桁 | `26` = 2026 |
+| `MR` (MINOR) | 月 × 10 + 月内リリース番号 | `31` = 3月1回目, `32` = 3月2回目 |
+| `DDR` (PATCH) | 日 × 10 + 日内リリース番号 | `291` = 29日1回目, `292` = 29日2回目 |
+
+**例**:
+- `26.31.291` — 2026年3月29日、月内1回目、日内1回目
+- `26.31.292` — 同日2回目
+- `26.32.301` — 3月30日、月内2回目（機能追加）
+- `26.41.11` — 4月1日、月内1回目
+
+**CRITICAL**: MINOR を上げても月が変わったことを意味しない。月内の機能リリース回数を表す。
+
+### ライセンス
+
+- **smalruby3 本体**: MIT
+- **依存ライブラリ**（LICENSE の Third-Party Notices に記載）:
+  - ruby-sdl2: LGPL-3.0（gem 依存、動的リンク）
+  - resvg: MPL-2.0（Rust crate、smalruby3_imageutil にコンパイル）
+  - rsdl: Ruby's License（smalruby3_launcher の元コード）
+
+### gem ビルド・公開
+
+```bash
+# ビルド
+cd ruby/smalruby3
+gem build smalruby3.gemspec
+
+# 公開
+gem push smalruby3-YY.MR.DDR.gem
+```
+
+**注意**: `spec.files` は `Dir[...]` で列挙し、`ext/smalruby3_imageutil/target/` を除外する。
+
 ## Tech Stack
 
 - **Ruby**: 3.3+ (`rbenv local 3.3.9` が設定済み)
diff --git a/.claude/skills/gem-release/SKILL.md b/.claude/skills/gem-release/SKILL.md
new file mode 100644
index 00000000000..29949777a2e
--- /dev/null
+++ b/.claude/skills/gem-release/SKILL.md
@@ -0,0 +1,98 @@
+---
+name: gem-release
+description: "Build and release the smalruby3 gem to RubyGems. Handles version bump, gem build, and push."
+argument-hint: "[version e.g. 26.31.291]"
+---
+
+# /gem-release - smalruby3 gem リリース
+
+smalruby3 gem をビルドして RubyGems に公開する。
+
+## 引数
+
+- `$ARGUMENTS` — リリースバージョン（例: `26.31.291`）
+
+バージョン未指定の場合は `ruby/smalruby3/lib/smalruby3/version.rb` の現在のバージョンを使用する。
+
+## バージョニング: YY.MR.DDR
+
+| フィールド | 意味 | 例 |
+|-----------|------|-----|
+| YY (MAJOR) | 年の下2桁 | 26 = 2026 |
+| MR (MINOR) | 月 × 10 + 月内リリース番号 | 31 = 3月1回目 |
+| DDR (PATCH) | 日 × 10 + 日内リリース番号 | 291 = 29日1回目 |
+
+## 手順
+
+### Step 1: バージョン確認・更新
+
+1. `ruby/smalruby3/lib/smalruby3/version.rb` の現在のバージョンを読む
+2. 引数でバージョンが指定されている場合:
+   - `version.rb` を更新する
+   - YY.MR.DDR 形式であることを検証する
+3. 引数がない場合:
+   - 現在のバージョンをそのまま使用する
+   - ユーザーに確認: 「バージョン X.XX.XXX でリリースしますか？」
+
+### Step 2: テスト・lint
+
+```bash
+docker compose run --rm smalruby3 bash -c "bundle exec standardrb && bundle exec rake test"
+```
+
+テストが失敗した場合はリリースを中止する。
+
+### Step 3: gem ビルド
+
+```bash
+cd ruby/smalruby3
+gem build smalruby3.gemspec
+```
+
+ビルド成功を確認し、`gem specification smalruby3-VERSION.gem` で metadata を表示する。
+
+### Step 4: ユーザーに最終確認
+
+以下を表示してユーザーに確認を求める:
+- バージョン
+- gem ファイル名
+- gem に含まれるファイル数
+- 依存関係
+
+「`gem push` を実行してよいですか？」と確認する。
+
+### Step 5: gem push
+
+**IMPORTANT**: この手順はユーザーの明示的な承認後のみ実行する。
+
+```bash
+cd ruby/smalruby3
+gem push smalruby3-VERSION.gem
+```
+
+初回は認証情報の入力が必要。ユーザーに `! gem push smalruby3-VERSION.gem` の実行を案内する（インタラクティブ認証が必要なため）。
+
+### Step 6: タグ付け・コミット・プッシュ
+
+1. バージョン更新がある場合はコミット:
+   ```bash
+   git add ruby/smalruby3/lib/smalruby3/version.rb
+   git commit -m "release: smalruby3 vVERSION"
+   ```
+
+2. タグを作成:
+   ```bash
+   git tag smalruby3-vVERSION
+   ```
+
+3. プッシュ:
+   ```bash
+   git push origin HEAD
+   git push origin smalruby3-vVERSION
+   ```
+
+### Step 7: 完了報告
+
+- RubyGems の URL: https://rubygems.org/gems/smalruby3/versions/VERSION
+- タグ: `smalruby3-vVERSION`
+- 次回リリース時のバージョン例を提示
diff --git a/.github/workflows/ci-ruby.yml b/.github/workflows/ci-ruby.yml
index 093bc207bd0..214a9c0c0f2 100644
--- a/.github/workflows/ci-ruby.yml
+++ b/.github/workflows/ci-ruby.yml
@@ -36,8 +36,12 @@ jobs:
         run: bundle exec standardrb
 
   unit-test:
-    name: "[smalruby3] Unit Tests"
+    name: "[smalruby3] Unit Tests (Ruby ${{ matrix.ruby }})"
     runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        ruby: ['3.3', '3.4', '4.0']
     defaults:
       run:
         working-directory: ruby/smalruby3
@@ -51,7 +55,7 @@ jobs:
         uses: dtolnay/rust-toolchain@stable
       - uses: ruby/setup-ruby@v1
         with:
-          ruby-version-file: ruby/smalruby3/.ruby-version
+          ruby-version: ${{ matrix.ruby }}
           bundler-cache: true
           working-directory: ruby/smalruby3
       - name: Compile native extension
diff --git a/.github/workflows/gem-build-check.yml b/.github/workflows/gem-build-check.yml
new file mode 100644
index 00000000000..08edc884e99
--- /dev/null
+++ b/.github/workflows/gem-build-check.yml
@@ -0,0 +1,124 @@
+name: Gem Build Check (cross-platform)
+
+on:
+  workflow_dispatch:
+    inputs:
+      ruby-version:
+        description: 'Ruby version to test'
+        required: false
+        default: '3.4'
+        type: choice
+        options:
+          - '3.3'
+          - '3.4'
+          - '4.0'
+
+concurrency:
+  group: "${{ github.workflow }} @ ${{ github.head_ref || github.ref }}"
+  cancel-in-progress: true
+
+jobs:
+  build-linux:
+    name: "Build gem (Linux / Ruby ${{ inputs.ruby-version || '3.4' }})"
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: ruby/smalruby3
+    steps:
+      - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5
+
+      - name: Install SDL2 development libraries
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y libsdl2-dev libsdl2-image-dev libsdl2-mixer-dev libsdl2-ttf-dev
+
+      - name: Install Rust toolchain
+        uses: dtolnay/rust-toolchain@stable
+
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ${{ inputs.ruby-version || '3.4' }}
+          bundler-cache: true
+          working-directory: ruby/smalruby3
+
+      - name: Build gem
+        run: gem build smalruby3.gemspec
+
+      - name: Install gem locally
+        run: gem install --local smalruby3-*.gem --no-document
+
+      - name: Compile native extension
+        run: bundle exec rake compile
+
+      - name: Run tests
+        run: bundle exec rake test
+
+      - name: Verify smalruby3 loads
+        run: ruby -r smalruby3 -e 'puts "smalruby3 v#{Smalruby3::VERSION} loaded OK"'
+
+      - name: Upload gem artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: smalruby3-gem-linux
+          path: ruby/smalruby3/smalruby3-*.gem
+
+  build-windows:
+    name: "Build gem (Windows / Ruby ${{ inputs.ruby-version || '3.4' }})"
+    runs-on: windows-latest
+    defaults:
+      run:
+        working-directory: ruby/smalruby3
+    steps:
+      - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5
+
+      - name: Install SDL2 via MSYS2
+        uses: msys2/setup-msys2@v2
+        with:
+          msystem: UCRT64
+          update: true
+          install: >-
+            mingw-w64-ucrt-x86_64-SDL2
+            mingw-w64-ucrt-x86_64-SDL2_image
+            mingw-w64-ucrt-x86_64-SDL2_mixer
+            mingw-w64-ucrt-x86_64-SDL2_ttf
+
+      - name: Install Rust toolchain
+        uses: dtolnay/rust-toolchain@stable
+
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ${{ inputs.ruby-version || '3.4' }}
+          bundler-cache: true
+          working-directory: ruby/smalruby3
+
+      - name: Add MSYS2 to PATH
+        shell: bash
+        run: echo "C:/msys64/ucrt64/bin" >> "$GITHUB_PATH"
+
+      - name: Build gem
+        run: gem build smalruby3.gemspec
+
+      - name: Compile native extension
+        shell: bash
+        env:
+          CPATH: C:/msys64/ucrt64/include
+          LIBRARY_PATH: C:/msys64/ucrt64/lib
+          PKG_CONFIG_PATH: C:/msys64/ucrt64/lib/pkgconfig
+        run: bundle exec rake compile
+
+      - name: Run tests
+        shell: bash
+        env:
+          CPATH: C:/msys64/ucrt64/include
+          LIBRARY_PATH: C:/msys64/ucrt64/lib
+          PKG_CONFIG_PATH: C:/msys64/ucrt64/lib/pkgconfig
+        run: bundle exec rake test
+
+      - name: Verify smalruby3 loads
+        run: ruby -r smalruby3 -e "puts \"smalruby3 v#{Smalruby3::VERSION} loaded OK\""
+
+      - name: Upload gem artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: smalruby3-gem-windows
+          path: ruby/smalruby3/smalruby3-*.gem
diff --git a/.github/workflows/release-gem.yml b/.github/workflows/release-gem.yml
new file mode 100644
index 00000000000..e40fa4c18ee
--- /dev/null
+++ b/.github/workflows/release-gem.yml
@@ -0,0 +1,98 @@
+name: Release - smalruby3 gem
+
+on:
+  push:
+    tags:
+      - 'smalruby3-v*'
+
+permissions:
+  contents: write
+  id-token: write
+
+jobs:
+  test:
+    name: "[smalruby3] Test before release"
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        ruby: ['3.3', '3.4', '4.0']
+    defaults:
+      run:
+        working-directory: ruby/smalruby3
+    steps:
+      - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5
+      - name: Install SDL2 development libraries
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y libsdl2-dev libsdl2-image-dev libsdl2-mixer-dev libsdl2-ttf-dev
+      - name: Install Rust toolchain
+        uses: dtolnay/rust-toolchain@stable
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ${{ matrix.ruby }}
+          bundler-cache: true
+          working-directory: ruby/smalruby3
+      - name: Compile native extension
+        run: bundle exec rake compile
+      - name: Run lint
+        if: matrix.ruby == '4.0'
+        run: bundle exec standardrb
+      - name: Run unit tests
+        run: bundle exec rake test
+
+  release:
+    name: "[smalruby3] Publish to RubyGems"
+    needs: test
+    runs-on: ubuntu-latest
+    environment: rubygems
+    defaults:
+      run:
+        working-directory: ruby/smalruby3
+    steps:
+      - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5
+
+      - name: Extract version from tag
+        id: version
+        run: |
+          VERSION="${GITHUB_REF_NAME#smalruby3-v}"
+          echo "version=${VERSION}" >> "$GITHUB_OUTPUT"
+          echo "Releasing smalruby3 v${VERSION}"
+
+      - name: Verify version matches
+        run: |
+          GEM_VERSION=$(ruby -r ./lib/smalruby3/version -e 'puts Smalruby3::VERSION')
+          TAG_VERSION="${{ steps.version.outputs.version }}"
+          if [ "$GEM_VERSION" != "$TAG_VERSION" ]; then
+            echo "::error::Version mismatch: gem=$GEM_VERSION tag=$TAG_VERSION"
+            exit 1
+          fi
+
+      - name: Install SDL2 development libraries
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y libsdl2-dev libsdl2-image-dev libsdl2-mixer-dev libsdl2-ttf-dev
+
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version-file: ruby/smalruby3/.ruby-version
+          bundler-cache: true
+          working-directory: ruby/smalruby3
+
+      - name: Build gem
+        run: gem build smalruby3.gemspec
+
+      - name: Push to RubyGems (Trusted Publishing)
+        uses: rubygems/release-gem@v1
+        with:
+          working-directory: ruby/smalruby3
+
+      - name: Create GitHub Release
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          gh release create "$GITHUB_REF_NAME" \
+            --repo "$GITHUB_REPOSITORY" \
+            --title "smalruby3 v${{ steps.version.outputs.version }}" \
+            --generate-notes \
+            smalruby3-${{ steps.version.outputs.version }}.gem
diff --git a/ruby/smalruby3/LICENSE b/ruby/smalruby3/LICENSE
new file mode 100644
index 00000000000..43a49a09401
--- /dev/null
+++ b/ruby/smalruby3/LICENSE
@@ -0,0 +1,49 @@
+MIT License
+
+Copyright (c) 2024-2026 Kouji Takao
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+
+---
+
+## Third-Party Notices
+
+This software depends on the following third-party libraries with their
+respective licenses:
+
+### ruby-sdl2 (LGPL-3.0)
+
+Ruby bindings for SDL2.
+https://github.com/ohai/ruby-sdl2
+Licensed under the GNU Lesser General Public License, Version 3 (LGPL-3.0).
+Used as a gem dependency (dynamic linking).
+
+### resvg (MPL-2.0)
+
+An SVG rendering library used in the smalruby3_imageutil native extension.
+https://github.com/nickel-rb/resvg
+Licensed under the Mozilla Public License, Version 2.0 (MPL-2.0).
+Compiled as a Rust crate dependency.
+
+### rsdl (Ruby's License)
+
+macOS SDL2 wrapper command. The smalruby3_launcher native extension is
+derived from rsdl.
+https://github.com/knu/rsdl
+Licensed under Ruby's License (BSD-2-Clause compatible).
diff --git a/ruby/smalruby3/README.md b/ruby/smalruby3/README.md
new file mode 100644
index 00000000000..94f15d1132a
--- /dev/null
+++ b/ruby/smalruby3/README.md
@@ -0,0 +1,47 @@
+# smalruby3
+
+Scratch-compatible Ruby runtime for [Smalruby](https://smalruby.jp).
+
+A Ruby gem that provides a Scratch VM-compatible runtime, allowing Ruby scripts generated by [smalruby3-editor](https://github.com/smalruby/smalruby3-editor) to run natively on the desktop using SDL2.
+
+## Requirements
+
+- Ruby >= 3.3
+- SDL2 (system library)
+- Rust toolchain (for building native extensions)
+
+## Installation
+
+```bash
+gem install smalruby3
+```
+
+## Usage
+
+```bash
+smalruby3 your_script.rb
+```
+
+Or use directly in Ruby:
+
+```ruby
+require "smalruby3"
+
+# Your Smalruby script here
+```
+
+## Development
+
+```bash
+git clone https://github.com/smalruby/smalruby3-editor.git
+cd smalruby3-editor/ruby/smalruby3
+bundle install
+bundle exec rake test
+```
+
+## License
+
+This software is licensed under the [MIT License](LICENSE).
+
+See the [LICENSE](LICENSE) file for details, including third-party notices for
+ruby-sdl2 (LGPL-3.0), resvg (MPL-2.0), and rsdl (Ruby's License).
diff --git a/ruby/smalruby3/lib/smalruby3/version.rb b/ruby/smalruby3/lib/smalruby3/version.rb
index 8b0e623997f..18492b8f733 100644
--- a/ruby/smalruby3/lib/smalruby3/version.rb
+++ b/ruby/smalruby3/lib/smalruby3/version.rb
@@ -1,5 +1,9 @@
 # frozen_string_literal: true
 
 module Smalruby3
-  VERSION = "0.1.0"
+  # Versioning scheme: YY.MR.DDR
+  #   YY  = year (last 2 digits, e.g. 26 = 2026)
+  #   MR  = month * 10 + monthly release number (e.g. 31 = March, 1st release)
+  #   DDR = day * 10 + daily release number (e.g. 291 = 29th, 1st release)
+  VERSION = "26.31.291"
 end
diff --git a/ruby/smalruby3/smalruby3.gemspec b/ruby/smalruby3/smalruby3.gemspec
index 1c2bd91fefc..315efc8031e 100644
--- a/ruby/smalruby3/smalruby3.gemspec
+++ b/ruby/smalruby3/smalruby3.gemspec
@@ -17,6 +17,7 @@ Gem::Specification.new do |spec|
   spec.required_ruby_version = ">= 3.3"
 
   spec.files = Dir["lib/**/*.rb", "lib/**/*.json", "ext/**/*.{rb,rs,toml,in,c.in}", "exe/*", "assets/**/*", "LICENSE", "README.md"]
+    .reject { |f| f.start_with?("ext/smalruby3_imageutil/target/") }
   spec.bindir = "exe"
   spec.executables = ["smalruby3"]
   spec.require_paths = ["lib"]
@@ -27,4 +28,5 @@ Gem::Specification.new do |spec|
 
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = "https://github.com/smalruby/smalruby3-editor/tree/develop/ruby/smalruby3"
+  spec.metadata["changelog_uri"] = "https://github.com/smalruby/smalruby3-editor/releases"
 end