Yes, in theory any CI/CD (whether Github, Gitlab, Jenkins etc,) is just a shell script with some warpping. But this wrapping matters because it's convenience: how you integrate with the repo, how you deal with variables, secrets, caching, deployment security and so on. Some people roughly figured this out and proposed some ways which other people learned, so switching is always a bit of a pain in various ways. But it is definitely possible - I did quite a lot of pipeline migration in my life and this is definitely not a blocker for a project of small to medium to biggish-size.