won a game, did not receive game properly.. about 7 days ago..

You could leave a comment in the giveaway so they get a notification and know theres something wrong. if their profile is public you could comment there either. otherwise mark not received and wait if he contacts you.

Check your library again , search the name of giveaway
Sometimes steam has bug, when you activate key too fast , steam notification the key has already been redeemed

after you follow the advice posted by others here, mark as not received. 🤷
and since you got ignored, decline their request to delete the giveaway if offered. ^^

already 7 days, ignored by creator
just mark "not received"

in my case it's the opposite though
i make GA, winner just mark "not received", no response whatsoever. not even complaining
only know after some weeks too (i dont think there's notification about not received)
but whatever, doesnt matter if i lose my max ga lol (still irritated about it though)

Just to add some hope to this thread. I've won a GA which looked like the gifter was ignoring a bunch of people. Turns out the gifter hadn't logged in for well over a week. Problems were promptly resolved after the gifter logged in.

Go ahead and mark the GA not-received, you can change it later. Check the gifter's profile to see the last time they logged in. They might not have seen your message yet. If they're acting in bad faith, the system will catch up to them, but most people are just dealing with other things in life and had an honest hiccup (like getting a bad key from a reseller, or making a copy/paste error and accidentally sending the same key to both winners). Give it some time and I bet the odds that things will work out amicably.

I see you've commented on the giveaway letting the GA creator know that the key was already used and you've marked not received. I'd post one more comment on the giveaway asking if they could provide you with a working key. Doing this may end up having a good outcome where you get a key that works and they get a received status. What's the worst that could happen?

The first step in any giveaway win process, is to communicate in the giveaway's comment section. This not only is by far the most reliable method for communicating with the GA creator, but it also leaves an easy-to-reference log for staff [which can avoid certain potential issues].

Regardless of whether you're winner or creator, it's considered impolite to add on Steam without any warning [eg, such as putting in the description "This is a steam gift, so I'll need to add you"], as it's rather intrusive and spammy in situations where it's unnecessary. However, if you're a winner, and there's a clear issue, and it's been some days since it was addressed, it's likely that most would find the action excusable by that point.

Once you've gone past those steps (as you've indicated you have), there's really only two options:

1. Mark the giveaway "Not Received". This may close off future chances at that particular win, but will immediately unlock your ability to start entering giveaways for that particular game again. If the game is commonly offered at low entry count, this is usually an easily recommendable choice.
2. Leave the giveaway feedback unmarked. This is considered polite to the GA creator, yet also puts some pressure on them to resolve the matter. If the game in question is rarely found on SG (or if you've a decent relationship with the individual in question), this option is definitely the most favorable. In the case of a rare game, you can simply mark Not Received whenever the game shows up again, and then immediately enter for it. You lose nothing by leaving the game unmarked.