Yes, GitHub pages are vulnerable to (sub)domain takeovers too [1], but GitLab has a couple of specific characteristics that make matters worse.
Looking over how GitLab handles setting up custom domains[1], it's pretty clear they were affected by that. I thought it was pretty much decided that's more a problem with the Baseline Requirements than with individual service providers like GitLab though. Mozilla even went so far as to forbid CAs from using two of the Baseline Requirement validation methods as a result of that vulnerability[2]. Assuming the CAs comply this shouldn't be an issue anymore, right?
Or were you referring to something else?
[1]: https://docs.gitlab.com//ce/user/project/pages/introduction....
[2]: https://groups.google.com/d/msg/mozilla.dev.security.policy/...