What happens if you purchase the upgrade edition for a game that you don't already own? Stasis is available for 30000 XP but the upgrade edition is also on sale for $2.29. Please don't give me an answer to just purchase it after getting Stasis. I know I can do that but I'd rather the 2300 XP (or 2530 XP with wallet) go towards getting Stasis rather than wasting it and purchasing extra.

I want to know if it will cause some big problem or if it will simply linger on my account till I get Stasis, or will Gog give me a code to redeem instead?

Well, if the upgrade is on a good offer while the main game isn't, you could speculatively go for it, then wait for the game itself to come on sale.

But I don't think I've ever seen a game upgrade be on sale without it's host on sale at the same time!

Is that upgrade even worth it? Serious question, I'm not trying to be a jackass.

I picked up Stasis last night with my XP and honestly the upgrade doesn't look like much.

Ahh thanks. :) Honestly I didn't think about that as an option.

I don't think you can, at least I can't actually complete the transaction without owning the base game.
I think the same goes for soundtracks which are sold as 'DLC'.

Ah, so GOG has a "safety net" to stop you buying something you may not be able to use. Makes good sense.

Meh, whatever works.

I am sure the trick paladin181 mentioned do work.
I have bought time-limit free DLCs for myself, and get the redeem code.
GOG system did not let me buy it directly, so I bought it as a gift.