So I think the music of the game is excellent but unfortunately this seems like one of the few games in all GOG cataloge that doesn't include the OST.

So is there any way to get it? I tried with a unity asset explorer but I only got a few sounds and trash. Anyone had better luck?

I'd even pay a few orens to get it but it is not an option either ;)

Isn't it from the previous games? try to check their soundtracks.

Not all of it. I think there are few tracks from Witcher 2 (Dwarven Stone upon Dwarven Stone ?), but there is also one from Witcher 3, namely "Merchants of Novigrad". There are however tracks that were not in any previous Witcher games I think. But all I know of Witcher 3 music is based on four tracks given to people who pre-ordered it, so maybe these "unknown" tracks come also from Witcher 3, but haven't been released yet.