There is an enemy speed issue with the initial release version, that is supposed to affect anything above normal difficulty. This was confirmed by Night Dive as the patch notes state this was fixed in Unofficial Update #1.
Now, on asking on the Steam forums when GOG would get the update, the Night Dive staff there mentioned that GOG support were on holiday so couldn't say when the patch would show up here. From that, I deem this to be a GOG issue rather than Night Dive at fault.
For this reason, I refrained from making the purchase here, and I may yet get it on Steam. Sure, I'd prefer a DRM free version, but I can always get one from here at a later date in some sale or other.