In a way AFAIK Gnome and Debian moved to GitLab, because, among other reasons, they did not have enough horse power to maintain their own solution - plethora of services. Of course if the GitLab would fold, they would maintain it, but I'm not sure they would be better off than before in case of work allocation at least.
If gitlab were to fold, they would probably move to whoever is the best candidate at the time.
While moving is very painful, the fact that git is distributed means that setting up camp somewhere else is pretty smooth. Even the headaches of the other stuff, such as issues et al, will probably be handled by whoever the new host is.
