Issue tracking is optional, if you disable it users wont be able to fill issues. You can easily ignore release system (by not using it) and I am not sure what you mean by authentication. As for pull requests, it is reasonable to expect contributors to submit them wherever your readme points to.

I don't mean to say that gnome should use github, I think that it is overall better if there is no single dominating company for open source. Just that organizing so that github is git hosting only is not that hard.