I've just spent the evening fighting Bundler and the way it represents git dependencies, but I finally have my #ruby project building using #guix, with a manifest that's imported from a Gemfile, similar to Bundix[1].
It's pretty hacky, but now that I've figured out enough to get our (fairly complex) project working I'm more confident that I can get this working more generally.
If I manage to clean up my patch allowing channels to add commands to Guix then I might even get to have a "guix ruby" command to convert a Gemfile (and/or Gemfile.lock) into a Guix manifest.