Yeah, it's almost certainly the network effect. Although poor GitLab isn't doing themselves any favors by picking what seems to be the slowest web framework one can possibly imagine
But, anytime I am empowered to pick, I'm going to pick GitLab 100% of the time because it has every feature that I care about and "being popular" isn't a feature that I care about
I think the GitHub brand is still stronger and people just don't "care" about gitlab.