One of the founding principles of open source is that you can take a copy of someone's work and make changes to it (as long as you continue to make the changes public).
Github isn't open source so the analogy doesn't quite hold, but they're creating something and giving it to the community (as well as building a business).
> One of the founding principles of open source is that you can take a copy of someone's work and make changes to it (as long as you continue to make the changes public)
Also as long as you acknowledge it. It’s acceptable to copy a FOSS project and make a new one on top of it but you have to mention it; you can’t just create a copycat and claim you invented everything by yourself, especially if you start making money off it (well some licences let you do that but that’s not ethical).
The guys at GitLab are usually quite open about this sort of thing (having copied this and that, etc). I don't think they've ever claimed to have invented anything they have not.
We indeed try to be open about it. Let us know if we should improve the attribution somewhere, in this thread we already made a fix in https://news.ycombinator.com/item?id=11432400
Github isn't open source so the analogy doesn't quite hold, but they're creating something and giving it to the community (as well as building a business).