GitLab team member here. We have Code Owners [1] and recently introduced Merge Request Reviewers [2]. Our team is working on the author-review handoff [3] as part of their work to improve the overall merge request reviewer experience [4]. I'm sure they would appreciate any detailed feedback you could share on the epics linked below.
Code Owners don't work. If you assign a team as the owner it doesn't do anything unless the team itself is also added to the project. Further it doesn't seem to properly notify people either. We've fallen back to emailing everyone manually whenever a pull request is submitted.