I just see that Amnesia was for sale on Gog, so I had to buy it. I own a Steam copy of it, with the "Justine" expension. My question is is there a way to play Justine on the GoG version?...
You'll be out of luck if the expansion comes with its own executable to start it or alters the original executable, as it will rely on Steam. If the expansion only shows in-game and is started with the game's standard executable, you could try coping the additional files into the GOG version's folder structure.
I don't really want to download the Steam version again, just to "move" the files. I bought the GoG version to be able to play it anytime, on any machine, and installing steam clients, downloading the game a second time and moving the files is a bit cumbersome.