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.

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).

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.

One issue I have with them is that they pretty much had all the features I use a few years after they started, and they have, for the most part, just kept adding new ones of dubious value instead of polishing the core ones.

Could be, but that probably makes their product bad, not simple :-)