Are there any substantial changes beyond Steam achievements? Because these will never come to GOG (or any other service besides Steam).
More difficulty levels were mentioned, but I don't really see the need for that to be honest.

20th Anniversary Edition has been released on DotEmu
http://www.dotemu.com/en/download-game/5391/another-world-20th-anniversary-edition-pc

There is one thing in the bonuses of the 20th Anniversary Edition that GOG doesn't supply. The Amiga version as .adf files is tucked away in Bonus\rom Amiga\

So if you happen to have an Amiga emulator or even a real Amiga lying around, you can get the real original experience. Works nicely on my Amigas ;)

Yup, The 20th ani Version is here. I just installed and played a little. I don't see any difference, but this is it.
Great service by GOG as always!

I made the terrible mistake and installed the German version. Can't remember when I last saw such a bad translation before. It almost looks like as if the person in charge used a translation program and took the first result he got.
Good thing it's just the menu and options...

And about the differences: The sound effects seem to be slightly different. Almost like they're more "refined". I could be wrong though ;-)

Edit: Okay, it isn't that bad on the second view. It was just the initial shock I guess!