I want to say it's the freedom to self-host, but really it's the CI.

Throwing a .gitlab-ci.yml file into my repo is by far the easiest way to get CI/CD incorporated into a product. The configuration of test runners (or lack thereof) is both a breeze and extremely powerful.

I'm not even talking about the auto-devops thing that I haven't tried; writing your own gitlab-ci file worked in frictionless ways that CircleCI, Travis, and (especially) Jenkins didn't.