GitHub isn't open-source, so while you're correct about what open source licenses allow, that's not really tangible to the complaint that they ripped off GitHub.
I see. It seems you are right. Github is closed source, Gitlab CE is open source, Gitlab EE is closed source.
Reframing the question:
Isn't ripping of and competing with the implementation of an idea is just normal business. Or is there something like gentleman rule where you should not copy business ideas as fast as you can?
Just a small addition - GitLab EE is source-visible and proprietary licensed. Everyone can browse the source code over at [1]
The initial GitLab UI felt a lot like a direct copy from GitHub.
I'd consider "normal business" to be some combination of creating new benefits for your service that your competitors don't have and replicating the kinds of benefits they add, hopefully doing them better.
As an example: both GitHub and GitLab have recently launched features in the "security monitoring for your projects" space, but they're both independently good features. Had GitLab launched the feature and then GitHub just copied all the css/js and stood up a clone of it, I'd consider that to be highly questionable.
Can you elaborate on that?
Because ripping off and competing against original developers is what open source specially allows. That's one of it's benefits of open source. If someone can improve and and do cheaper, they should do it. I don't see any moral implications.
Software business service business not manufacturing business. (at least 99%). If you are against ripping off, maybe put more restrictive license or go closed source. Then you can leverage accumulating IP from work you do.