GitLab is used only for code review, issue tracking and project management. Canonical locations for source code are still https://gitweb.torproject.org/ https://git.torproject.org/ and git-rw.torproject.org.
MB asked me for this a while ago. It would help incrementally with the RAM consumption and startup latency issues that we have with some domains.
It could be implemented by just sticking the domain in the HSTS preload list, or by emulating that behaviour using our existing API hooks.