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.
@steph and the OTF Learning Lab are working on this content. This ticket will track implementation.
To enable design management, you'll need to meet the requirements. If you need help, reach out to our support team for assistance.