I really want to buy this, but lack of soundtrack downloads like some other games on GOG, make me avoid this. It's just too good to ignore this masterpiece's soundtrack.

May have something to do with the fact that it's impossible to simply export the music from the game as it's based on a dynamic tracker system - even Jesper Kyd, the composer, apparently had problems turning it into an album since the Hitman 1 soundtrack was released later than that of Hitman 2 (and maybe even Hitman: Contracts) and differs quite a bit from the music actually heard in the game. And I guess that all the rights to the album belong to Kyd, not the publisher. That's probably why they didn't include a soundtrack with the game.

Besides: considering that I paid 25€ for the soundtrack I'd feel royally pissed if it was made available along with the game for 6$.

Would you feel that it hadn't been worth the 25€ in the first place then?
I find that hard to believe, the game's musical score is absolutely great. For this reason, I support the OP's request.

I occasionally buy games on GOG solely because they come with the soundtracks, and Jesper Kyd is a master. I would love to add any of his work to my collection while supporting GOG.

You can purchase it separately, mate. GOG couldn't have included it without raising the price of the game significantly.

http://www.jesperkyd.com/