The Mac version hasn't been released, and we don't yet have a release date for that platform. Larian have only said they're working on it and hope to announce other platforms soon.
Apparently the same Steam/GoG keys are valid for PC and Mac, so even if you don't have a PC that can run the game, if you have the game on Steam and some way of accessing and playing GeForce Now then I'm not sure whether that might be a potential workaround for you while you wait for the Mac version?
