i won game key doesnt work.

Dont mark it as received and contact the GA creator.

Why don't you try contacting the GA creator (maybe comment on the GA?) first so that they can see if they made a mistake or they can get a working key before that? If that happened to me, I'd appreciate it if the person let me know so that I can fix the problem.

Comment on the giveaway and tell the creator about it. Maybe they have another key or want to replace it later. They might potentially also ask you if you're ok with deleting the giveaway.

If they do not respond as you said just mark it not received after a week 😉