The GitHub mirror for the GNOME repositories was set up back when GNOME’s infrastructure was based on cgit. It was created mainly for one reason: allowing people without a GNOME account to easily set up a forked repository for their work, before submitting patches for review in Bugzilla.
To avoid confusing contributors, both pull requests and issues were disabled in the GitHub mirror.
Since then, GNOME has switched to GitLab, which makes a lot of the reasoning for a GitHub mirror moot:
- you can easily create forked repositories and open merge requests
- you don’t need a GNOME account to do so
Still, it’s easy to see that anybody with a GitHub account, and familiar with the GitHub workflow, might just want to use the mirror to contribute to GNOME projects. For that reason, the GitHub mirror runs a script that intercepts pull requests and redirects them to GitLab.
My personal recommendation is not to set up a GitHub mirror for a smaller project. It works on GNOME because GNOME has a fair amount of repositories, and you can easily set up an organisation for it. Plus, GitHub deals with that kind of organisational mirroring pretty easily.
For a smaller project, I’d either recommend switching to GitHub or, if politics are an issue, to GitLab. I would not recommend SourceForge as a code hosting service in 2021, even with all the changes that their new owners introduced after their spectacular set of boneheaded moves 10 years ago, where they were more of a spamware server than anything else.
The benefit of GitHub is, of course, not in the workflow—as any code forge is essentially reimplementing it—but in:
- the reliability of the service
- the integration with cloud building for CI/CD pipelines
- the social and outreach aspects of coding open source software
None of those are going to be useful to you if you’re just hosting a mirror—except, maybe, the outreach part, which requires some integration tool to ensure that people using the mirror can easily submit code contributions through GitHub, or, at least, be pointed to the right place with minimal cost. Something that, I suspect, is the reason why one of your developers is asking for a wholesale move to GitHub in the first place.