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.
https://gitweb.torproject.org/atlas.git/tree has no license file.
My only hint is at
where it says that "it's also released under BSD 2 clause license".
But I don't see anything in the source tree to back that up.