gitlab.gnome.org. It has been only a couple of
weeks since librsvg moved to gitlab, and I've
already received and merged two merge requests. (Isn't it a bit
weird that Github uses "pull request" and Everyone(tm) knows the PR
acronym, but Gitlab uses "merge request"?)
Notifications about merge requests
One thing to note if your GNOME project has moved to Gitlab: if you want to get notified of incoming merge requests, you need to tell Gitlab that you want to "Watch" that project, instead of using one of the default notification settings. Thanks to Carlos Soriano for making me aware of this.
Notifications from Github's mirror
The github mirror of git.gnome.org is configured so that pull requests are automatically closed, since currently there is no way to notify the upstream maintainers when someone creates a pull request in the mirror (this is super-unfriendly by default, but at least submitters get notified that their PR would not be looked at by anyone, by default).
If you have a Github account, you can Watch the project in question to get notified — the bot will close the pull request, but you will get notified, and then you can check it by hand, review it as appropriate, or redirect the submitter to gitlab.gnome.org instead.