Are you sure? Because, I just tried it in a personal repository at GNOME’s GitLab, and I saw no sign that’s the case.
By Merge Request Reviews I’m specifically referring to the functionality I linked to in my previous (longer) reply, which was finally released by the antispam gods. GitLab CE lets you comment on code lines, yes, but only individually — not as part of a combined review. That’s a premium feature.
The equivalent functionality in GitHub, however, is accessible to all users. I’ve found it extremely useful when giving feedback on PRs.