I put the first game in my cart then removed it. I debated for several minutes whether to add the two-pack to my cart. Who knows if the artbook and the OST will ever come to GOG? Since Steam has deep, deep sale right now, I got it there. If it's everything I heard, I will get a backup copy on GOG. If not, I won't miss the $5.56.

The GOG version has been outdated for two years now... Steam has v1.4.1.2

Oh! That's not good. Thank you for telling me.