So I got The Witcher 3 ages ago through GPU bundle, but both Galaxy and the GoG site refuse to acknowledge I already own it and keep pestering me to buy it again. Is there anything I can do to fix this? It's annoying AF.

Sigh... If that is annoying AF to you, simply ignore it. I'm sure you must have some more important issues to deal with.

GOG only knows what package (there are different ones) you bought on GOG, not through some third party GPU bundle. Besides that, there is absolutely no badgering involved; it's a mere advert for a package that GOG has no notion that you haven't bought yet. Jeez!

Sigh... if all you have to contribute is a sigh, simply don't post. I'm sure you must have more important issues to deal with. Funny how Steam has no trouble determining what I already have in my library whether bought from them or redeemed via code, but GoG has "no notion" about redeemed codes. It's a pretty simple check - if game in library then mark bought or something similar. Not rocket science.

And it's not just an eyesore (or a mere advert), it could cause me or someone else to buy it again while looking to pick up the expansions if not paying attention.

You do know that GOG Galaxy is still in beta?

And I'm providing feedback. Also, the issue isn't exclusive to Galaxy, it's on the GoG site as well (which most decidedly is NOT in beta), as I said in the OP. Reading comprehension - it's useful.

To me it sounded more like complaining than feedback, that's why I mentioned it. So you would like a random site on the internet to be able to just scan your installed software (in this case read "games" instead of "software"). Sure, what could possibly go wrong....

No you're not, you're whining and complaining. There is nothing constructive about your posts.