It's really "open core", but they use one code base, so commercial-licensed code is "available" but not under an open source license.

https://about.gitlab.com/handbook/marketing/strategic-market...

NOTE: I'm not associated with GitLab at all I just found the topic interesting. I don't know of many other projects that actually do things this way, with all the commercial licensed code mixed in with the open source code.

This was kind of an interesting speech I found searching around: https://www.heavybit.com/library/video/commercial-open-sourc...