Is there any way that you could add achievements to this game? Steam version have it, why gog shouldn't? It would be nice to get it.

You need to contact developer/publisher Beamdog about this. It's fairly common for games to be released here without achievements, for several reasons:
1. Developer laziness. It takes more than zero effort to implement this feature.
2. GOG's minuscule market share provides little pressure to provide this feature or updates to the game.
3. Out of GOG's user base, there's a chunk who never touch GOG Galaxy and just use offline installers.