In general, the foundations upon which it was built.

In specific, off the top of my head:

- you can't group repos

- CI/CD is far behind what GitLab offers

- Review apps (though I think now you can do this on GH with other tools)

- Can't mark PRs as drafts by default in GH

- better metric tracking, easier to see a "snapshot" of where something is at on GitLab

Tbf, GH may have closed some gaps over the years but when I was bouncing between the two GH always felt a lot more rigid and further behind than GL. It did have fantastic support for open source though, and has a few unique and fun things like the `io` site stuff. Overall though if I could choose which to work in for serious projects I'll always choose GL.

GitLab has a top of the class CICD system that is leaps and bounds over GitHub Actions. That alone makes it worth using GitLab.