Code review with GitLab?
Yes, it's possible. Gitlab has the concept of Discussions (either single comments or discussion threads), which you can have on a variety of development items/activities, including the code itself (in which case they effectively are code reviews):


  The ability to contribute conversationally is offered throughout
  GitLab.
  
  You can leave a comment in the following places:
  
  
  issues
  epics
  merge requests
  snippets
  commits
  commit diffs
  


But note that only merge request discussions are presently considered resolvable.

Even though the check for a merge request can be configured to be automated, the starting of a review/discussion itself (if/when needed) is a manual operation. From Threaded discussions:


  To start a threaded discussion, click on the Comment button toggle
  dropdown, select Start discussion and click Start discussion
  when you’re ready to post the comment.