GoG runs patches and new versions through its own QA before releasing them (which is not the case with Steam). This usually means that a patch release will take an additional day or two to appear, but the extra set of eyes going over things has proven valuable in the past since from time to time they've caught 'bad' patches that ended up live on Steam.
My only question is, how does GoG run QA on a patch for a game this large? Do they dig through the code? Do they play the game? It seems an odd practice, to me. At least for new games, anyway.