GitHub Actions is the same as everything else. It doesn't work any better or worse. But the UI is clunkier, you can do less with it than other systems, it lacked important features forever (still does). The only reason you think it's significantly better is psychological.
The "market" of actions is better than competitors because the incumbent gets an advantage, and it trades on the idea of a generic executable as a 3rd party plugin, unlike other systems that try to force you into a particular language or paradigm. But other systems do this too.
I build enough pipelines in GitLab way to be able to tell you that GitHub Actions is much easier for me to implement.
Its much closer than if you would have a second system like jenkins, i already have access to git and the packages.
I do use much more container images than i did 6 or 10 years ago, that might have something to do with it but nonetheless, i do prefer GitHub Actions.
I also stoped liking GitLab though. The Autodevops was horrendes, the security issues is concerning and plenty of ideas or features are half done. this is also something which changed. I liked GitLab much more than i do now. But looking at the year, i haven't looked at GitLab for at least 2 years now.
The "market" of actions is better than competitors because the incumbent gets an advantage, and it trades on the idea of a generic executable as a 3rd party plugin, unlike other systems that try to force you into a particular language or paradigm. But other systems do this too.