nicode: Well, one might ask someone with a Steam account to list and checksum the game folder.
Well, as GOG version is DRM free while STEAM version uses DRM I don't think you can compare it that easily. As the .exe itself will differ too.
Anyway, I wanted to wait to buy ELEX. I've been waiting to see, if people complain about the story getting lame while progressing in the story. Just like it happend in Risen, where the story was immersive, but then it began to rush just as PB didn't care to much about it anymore ... possibly they lost ideas or had to hurry because of the release. Never got used to R2 or R3 afterwards. Still unsure, but I was thinking about buying it anyway as people seem to be fine with the game for now.
But reading this topic and seeing there is no reply, neither by GOG or PB, it proves me too wait and not to trust into PB too much. The time where they could rely on my trust because of G1 & G2 is over for me since quite a while.