GOG will sell you Linux versions of games. In fact it doesn't care - once you own it you can download all of the game files for all the platforms it was released for. However, it only sells you what actually exists. It doesn't take the Windows version and run it in a Windows emulation layer like Steam has. If you want to run the Windows version emulated on Linux, you're on your own.
Not on your own; you're with a community that supports running GOG games on that Windows emulation layer. Heroic can run most GOG games very well. Steam is more reliable because it's Valve itself that's behind it, but Heroic is not that far behind.
