Never really got how gitlab got so big, what's their USP? To my knowledge they don't really have anything that is not reproducible rather easily.
On premise Git forge with integrated issue tracker, CI/CD platform, and probably other 20 development adjacent tools. It has a ton of features so for sure it's not easily reproducible.
The selling point would be vertical integration and that you don't need to stitch together 12 different SaaS CI products all attached to your source code, but just deal with one vendor (GitLab).