Guys, it's being sold seperately so GOG couldn't include it without raising the price! I know it's probably annoying, but I think in this case they tried to strike a balance.
Anyway, I get the feeling many of these soundtracks can be purchased on Bandcamp. The FTL one can, and I found this: http://dva2.bandcamp.com/album/botanicula
DVA are, incidentally, a pretty cool band, I recommend checking out Fonok and HU if you like quirky folk/pop.