Just dumping everything on a (git) hosting service won't achieve much. Authors need to be convinced to start using them or things will just get stale. We've had a long discussion about this sort of stuff before, which resulted in the current sitation, where more and more g3 mods are under external revision control. Not sure you can access this thread though:
http://gibberlings3.net/forums/index.php?showtopic=27649
Either way, anything on github can be mirrored automatically, once you know it exists.