Also through their site, using Humble.
http://www.zojoi.com/retroshadowgate/

I just got the last one, Uninvited. Oh well. If not added to gog that'll be pretty crappy.

Yeah, I don't really understand what they're trying to do with the "Steam exclusive for a limited time" thing. I guess to try and get more people to buy the new Shadowgate? But then the way I read it you don't get Uninvited.

I am tempted to get the bundle of all 3 from their website, but would much rather have it here. Why would they do this to GOG?