The CI solution in GitLab is inadequate for our needs; we require the flexibility and extensibility of Jenkins.
The review tool in GitLab has some significant gaps compared with standalone tools like ReviewBoard, and even compared with the one built into Bitbucket. It also has some baffling UX decisions, like the thumb/emoji buttons and the approve/merge buttons which appear in the exact same location as each other. Seriously, just scrap what you have and copy Bitbucket's review interface.