I don't know the exact specifics, but I would imagine Gitlab has no chance of competing with GitHub, so they've made the decision to focus on a specific type of

I don't know the exact specifics, but I would imagine Gitlab has no chance of competing with GitHub, so they've made the decision to focus on a specific type of client. I think the closest alternative to GitHub that has active users is Codeberg, but it is a relatively small community. So, as you say, GitHub is pretty much the only game in town unless you've organized yourself to be part of IRC, mailing lists, and self-hosted version control systems.