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.
legacy/trac#22515 (moved) previous ticket for ESR 52.
https://lists.torproject.org/pipermail/tbb-dev/2018-May/000849.html http://f4amtbsowhix7rrf.onion/tor-browser-builds/