GCET is a tiny drop-in toolset for exporting content from a Craft CMS site.
Truth be told, there isn't much point in publicising this. In essence the entirety of GCET is made up of two "components", if you can even call them that.
First, Barrel, which is a little autoloading library.
Second, 2.fields.php, which contains the handlers for standalone fields.
Also note that the composer.json serves no purpose other than autocompletion within this repository.
The point of GCET is that you can copy the lib folder and place it in your Craft project's config folder for consumption within, for example, Element API. (for which an example was left in this repository, see here.)