More testing is certainly required, but it works right away after the most obvious adaptation, obviously relying on the Import from Url for the repo itself.
For now, all I've tested is just-created tiny repos with a couple of issues.
Please note that GitLab's Bitbucket import uses an instance-wide configuration that requires filesystem access, see GitLab documentation
The WIP marker is there in case I'd like to address some issues after further testing on my side, hence so that people don't merge right before I add a fix.
But I'm willing to ship this sooner than later, maybe as a beta in 0.6.1, so that it can be tested on realistic examples.