Case in point. Emacs moved from bzr to git, and they did it like that.

Official Git repo: https://savannah.gnu.org/git/?group=emacs

Unofficial github mirror repo: https://github.com/emacs-mirror/emacs

I don't think they set up the mirror, I think github just mirrored their repo.

GCC already has a GitHub mirror:

https://github.com/gcc-mirror/gcc

It seems out of sync though.

Worth pointing out though, it's far easier to keep a github mirror of a git repository up to date and correct than from other VCSes -- so even though people wanting to send GCC PRs on GitHub will be disappointed, it's still a win for discovery and searching.