Merge requests are a place to propose changes you've made to a project and discuss those changes with others
Interested parties can even contribute by pushing commits if they want to.
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.
Interested parties can even contribute by pushing commits if they want to.