The integrated CI is amazing, the Docker registry is nice, the issue boards are great, the saner permission model (you can add multiple people as repository owners without putting them in a team), these are the things I can recall off the top of my head.

Being able to have groups and organizations without the weirdness of GitHub.

GitHub's default path for organizations is that you are creating a business that will be very separate from your personal account.

GitLab's default path is that it's just a group. Maybe just a named group for related projects. Once you dig in though, GitLab's group admin is much better than GitHub in my opinion.

It's open source and self-hostable. Its delvelopment process is basically transparent, which is nice as well.

Other than that, their CI system is quite amazing!

Not the OP but I really like the free CI and container registry.