I was going to make a suggestion. I have not administered discourse or gitlab, but both have api-s, so it should be possible to make a script that runs once a day or a week and automatically pulls, for example, the emails of the Level 3 users from the forum. Then it looks in the gitlab database and if the user do not exist, create him with a random password. Which can be reset by the user.
…And then i saw the registrations are open 