You are mixing things up a little bit. I don’t blame you, because there are probably contexts where we can say “Steam is a platform” and it’s true. But in this case, it’s not, and we need analogy for this. Programmers love analogy.
Unity is a framework for making cross-platform games. In this sense, a “platform” is iOS, Android, Windows, MacOS, Linux distros, and the various consoles. Gems of War has been developed to target some, but not all, of these platforms. Gems of War is a game made with the Unity engine, and this is what determines what platforms it can run on.
Steam is a cross-platform video game store. It sells Gems of War. But there’s nothing Steam can do to make Gems of War run on a Mac.
It’s like Steam is Wal-Mart. Wal-Mart sells games for PS4 and XBox One. That doesn’t mean you’re going to play Final Fantasy XV on XBox One. It’s a platform-exclusive, being sold by a store that isn’t platform-exclusive.
So, to, does Steam sell many games that don’t target all platforms, or that target platforms Steam itself can’t support. Gems of War is distributed on PC by Steam, but that has little to do with whether it can or will support Mac.