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.
ref. tpo/web/newsletter#5 (closed). I believe it would make sense to point the newsletter entry in the services documentation to the readme document in the newsletter repo. The wiki pages there seem to be useful just for folks who publish the newsletter.