Open core is a business model not a license. You can download GitLab CE and use it under MIT license. You can use all the features it contains and do whatever you want with the code. Yes, GitLab Inc. has other products and some of them are paid, but that doesn't change the fact that GitLab CE is open source.

So if a company sells any closed source software, you don’t count any of their open source software as legitimate?

Not necessarily. The key is whether the closed-source software is an extension to the open-source software or something completely unrelated to it.