You can get witcher2.exe and paul.dll from GoG version, replace those files with the D2D version or the retail version, and there you go, you got the GoG version. The game will load much faster, and you'll gain more FPS if you have a dual core or a Quad core CPU computer. Go to gamecopyworld.com search for The Witcher 2, then select the link under 'Game Fixes', then download the file that says "done by GoG" And yes this works, I have the D2D version and my game is loading much faster, and I have gained about 10FPS on a quad core computer.
One of the developers posted in the comments section of The Witcher website supports doing this if you have a license to the game.
KarChee: Guys why didnt you buy it from GOG? Developers would get more money that way...
Do you use that same idea for everything you buy in life?