The original thread starter wrote he tried to add Spanish dialog sounds to the game, as the game has the option in the audio menu (but the audio is not included). He finally got the files (in the link provided) and extracted them in the "GameData\base" folder, located in the installation folder.

https://steamcommunity.com/app/6030/discussions/0/828939163752629904/

Can someone confirm it works for the GOG release as well?

It should. This is how the engine itself works (reading the contents of pk3 files in the base folder), which doesn't change between the GOG, Steam and retail versions.