Graphite supports bash and zsh tab completion. After installing Graphite for the first time, you can run (e.g. if using zsh, the default on Mac):
gt completion >> ~/.zshrc
For bash, you'd replace .zshrc with .bashrc or .bash_profile.
We don't currently support fish completions, but we're looking to — if you're passionate about it, maybe you can help us out with an open source contribution!