If by mobile you mean iPhone and iPad:
The C++ version works the same way it works on OS X, by creating a new Xcode project and importing Gosu via CocoaPods:
https://github.com/gosu/gosu/wiki/Getting-Started-on-OS-XThe Ruby version can be used on iOS with RubyMotion, which is free if you can live with a branded splash-screen. This (woefully incomplete/ugly) board game on GitHub has an example project in the
ios
folder:
https://github.com/jlnr/TrikusI honestly don't even know where to start documenting either process, so feel free to just ask questions on this board and I might turn the answers into Wiki pages.
Android is not supported right now. I guess it'd take one weekend to get the C++ version running smoothly on Android, and then a full week to wrap it for RubyMotion.