It didn't when last I checked, meaning the DLCs and soundtrack were impossible to purchase if you didn't already own vanilla, but I see that the OST has since been changed to a standalone item(which kind of suggests that's a "no").

If it still doesn't, that's unfortunately a pretty significant feature that the Steam copy has over GOG, however poorly implemented it is there.
I like the game a lot and would like to have the most feature-complete version, which I was hoping would be on GOG, for all the usual reasons.

Finally just bugged support about it and no, there's currently no getting Oldward on GOG.
Disappointing, but whaddayagonnado.