Ah! Got it to work.
For those wanting to know how it can be done:
Open your steam and click on adding a non-steam game, choose the game. Once that's done, go to your library, right click on Gabriel Knight, choose properties, a window should open. Leave it open for now and don't do anything.
Go to the folder where you installed GK and search for a shortcut called "Launch Gabriel Knight - Sins of the Fathers" (gog installs it automatically), right-click on it and choose "Properties", there, copy the text in the "target" window. Go to the steam window you previously opened and paste the text on the "target" window on said steam window.
Finally, go back to the folder where you installed GK, double click on "Launch Graphical Mode setup" and change the graphic mode to "Open GL".
The steam window can be a little slow and buggy at times, but it's the best I could do
Post edited November 26, 2013 by stika