I don't have as much experience with GitLab, but I actually think Github has a lot of tools in their PR system today for doing "early" or "work in progress" PRs incrementally. You can start a PR on an empty branch. You can apply a label "Work in Progress" and start discussion immediately. If you review a chunk of code the PR is pretty good about showing you what you've already reviewed versus what is new in the most recent update. If for some reason you feel a need to start over and force push an entirely new branch to the PR, Github's PR system handles it better than you would expect (and auto-closes old comments, etc).

I've found the tools are better than people expect them to be and the hard part for me is convincing junior developers that it is "okay" to start PRs "early", that they don't have to feel like they've finished a task to get comments/reviews on work in progress. There's a lot of embarrassment/group politics in the way sometimes from using "PR early" workflows.