I own the game on Steam (bought it before it was here :(...). Do I really need the game purchased on GOG in order to buy the soundtrack? It is on sale and currently cheaper on GOG.

GOG checkout automatically requires you to own a base game here to buy any DLC for it, even the DLC (like soundtracks and artbooks) that clearly *don't* need the base game at all. It's designed to prevent people from buying DLC for incompatible non-GOG versions, but it can backfire, as you've found.

I've heard some people say they got around it by contacting Customer Support and stating they wanted to buy independent DLC-X without owning unnecessary GAME-X, but I can't confirm that, as I've never tried it myself.

If you want to try that, I recommend you do it now, because Customer Support can sometimes take days to respond if they're getting a lot of support requests, and the new "in development" releases will likely be generating more than the usual support traffic.

I appreciate the reply and I will reach out to them if I end up wanting to purchase it.