Well, the service GoG.com advertises and is best setup to deliver is a unadulterated version of the game.
HOWEVER, on a game by game basis, getting what you want is very, very easy (in general...). In my case, all the games I've bought so far have very easy-to-use fanmade mods or patches to allow things like widescreen, and while GoG doesn't include that material you've probably noticed they often sticky "Mods for this game!" topics in every game board.
My advice is check the specific board of any game you're interested. If the mod you're looking for or fan-community is not listed, post anyway asking other users. Works most of the time. For instance, I am using a engine port for Descent 1+2 which GoG has no sticky about, but other users recommended. Gives me widescreen and was a simple cut/paste install.
Hope that helps.