We use GitLab because you can self-host it. GitHub is not an option for us, because we must maintain control of our data and source code. So GitHub was never really an option.

But we'd likely be using GitLab even if we didn't have this limitation. There's a bunch of features that make GitLab better for our specific team / situation. From my personal experience, GitLab CI is better than CI options available on GitHub. We use gitflow (ie. feature branches in the same repo), rather than pull requests. And it seems like GitLab supports this flow well.

I also tend to like GitLab issues much better than GitHub. The time tracking feature in EE is really useful (not sure if GitHub uses this). And issue boards across projects is also a very popular thing where I'm at, because we have lots of repos, rather than fewer big ones.

We also heavily use the deployment / environment features and built-in prometheus.