Does this mean I can ewrite a web app, put the source on GitLab and have GitLab run the web app (ideally with very minimal hassle)? If not, what does it mean? What is Gitlab letting me do now than (a) it didn't do before (b) Github doesn't do?
It means that a big part Gitlab's offering is now things like CI, whereas originally it was just code hosting.
https://gitlab.com/gitlab-org/gitlab/-/issues/213185
I'd join many participants in advising anyone to think very carefully before adopting GitLab.