GitLab was drastically cheaper, offering free private repos, and interesting features ahead of GitHub (although IMO always slightly less "sexy" than GitHub, using Ruby on Rails, etc.).
But at the time they gathered (1) serious funding money and (2) influx from MS-asylants their priorities started to change. But they were still the cheaper option for quite some time IIRC. The pandemic and the associated gold-rush/growth in IT pushed the dynamics over the edge I think.
Now their position is not really that different from GitHub's, and I think it is kind of a preference thing.
I can do with both, but I kind of still like the appeal and UX in GitHub. GitLab will always be in my heart, just like ever "Underdog" (even if that was a long time ago).
I could further see myself immediately falling for a third alternative, if it was sexy/unique enough with drastically better UX, and I think that is not even too far fetched.
But there is the thing, GitHub is a platform, not (just) a tool. GitLab still managed to take ground - kudos! That would be the hard part.