I don't know if I would call it GitLab's concept. Every place I have worked at that has used github has had the idea of a Work In Progress pull request. Its a pretty natural concept for an multi-member development team. So much so that all of the places I have used it have written explicit CI checks that would check for a label on the PR or the title to be in a given format and would fail to make sure you couldn't merge the branch until WIP was removed.