GitLab and gogs are both fine for getting 80% of the UI, but there are network effects with GitHub: that you're already signed in and have keys and such set up (although OAuth and API cleverness with keys can get you close), that your contributions across all projects show on your profile, that third-party tools like Travis know how to talk to your GitHub in particular, etc. And all sorts of little things, like that when you mention another project's PR in a commit, a note shows up on that PR. That's a lot of what makes GitHub nice to use, more than just the UI, and I suspect that's what pushed some of the Python core devs in favor of GitHub.
There's potentially a way to build a system that federates all of this, and then toss GitLab on top for the UI. A good standard would be whether a newcomer could submit a PR with only one more click (for the OAuth screen) than they would need on GitHub.
A good federated system would allow a fork from one host to end up on another and the PR would be done automatically after the very first PR was done with that extra click (and that extra click should do a bit of magic behind the scenes to send your key over to the other server together with your first sign-on on that server).
That way future interaction would hide the fact that the systems are in fact separate.
There's potentially a way to build a system that federates all of this, and then toss GitLab on top for the UI. A good standard would be whether a newcomer could submit a PR with only one more click (for the OAuth screen) than they would need on GitHub.