We currently mirror a bunch (all?) of extension and skin repositories, plus MediaWiki core, from gerrit to GitHub.
One nice thing about GitHub is the semantic code navigation capabilities, see documentation
That allows you to do something like, view https://github.com/wikimedia/mediawiki/blob/master/index.php#L28 and hover over wfEntryPointCheck() to see its usages and quickly navigate to where it's defined:
In the GitLab consultation, several people have expressed that they use GitHub for navigating code repositories since Gerrit's gittiles is kind of obscure to find and/or unintuitive to navigate.
My proposal would be to, in addition to our current mirroring, create a new repository on GitHub (e.g. wikimedia/mediawiki-all-skins-all-extensions) that contains a mirror of the latest core + all skins + all extensions, so that finding usages of a function call can be done from within the code browser of that repository. Even if we move to GitLab, we won't have the ability to do cross-repository searches due to limitations in the community edition of the software, so having this setup in GitHub seems like a useful thing to do.