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.
The Tor Browser User Manual on https://tb-manual.torproject.org/windows/en-US/ needs a section covering
a) where to find the source,
b) how it is licensed, and
c) where and how to report bugs.