This project is a framework, you may be interested in it only if you want to develop your own ebuild generator.
As user you may be interested in already implemented ones: gs-elpa and gs-pypi.
User instructions in gs-elpa are more complete, so consult them for how to use.
There is a lot of 3rd party software providers that resemble overlays or repositories of Linux distributions in some way. For example: pypi, CRAN, CPAN, CTAN, octave-forge, ELPA. It's clear that all this software available through different mechanisms (such as package.el for Emacs or pkg command in Octave) will never have separately maintained ebuilds in Gentoo tree or even in overlays. Installing such a software with its own distribution system does not seem like a good idea, especially if one needs to install it system-wide.
This project is aimed to create a framework for ebuild-generators for 3rd party software providers.
If you want to develop a new backend see developer's instructions.