Well, the key is for a different game that I already own. So steam won’t let me use it anyway. If you can’t find the correct key I guess you can request giveaway deletion from support?

LOL you're right and I already won that game 2 years ago from steamgifts. Already forgot about it was a long time ago. It's here:

https://www.steamgifts.com/giveaway/kd3jZ/alex-hunter-lord-of-the-mind-platinum-edition

I guess since this was a different version, Steamgifts let me enter the giveaway because on steam page it doesn't say I own this game aswell.

Then you can request a reroll in this case, since the key is unused.