I won, but the code's invalid.

Inform the sender by writing in the giveaway, which will PM him, and keep it marked unreceived.

Leave a message on the giveaway. The creator will receive a notification. Even if it's unresponsive, he have received the messages (apparently) so let's hope he read those. If the creator don't provide a working key, at the seventh day you will become God and you'll mark as 'not receive'.
Then you'll forgot about that seventh day crap and the "day of rest" and you will become human again and move on.

if you leave a comment on the giveaway he will get a notification next time he logs in steamgifts

If you are bored you can try out the 36 combinations a-z0-9 for the missing X (most likely at the end).

But since after 10 failed attemps steam puts you on cooldown for 20min-2h that might take a while if you are unlucky.

But might still be not enough since he might have missed one in the middle or something.

The creator has 7 days to deliver a working game. Feel free to click on "not received" if it's been more than 7 days.
If they contact you and give a working key you can change it to "received".
In the meantime you can enter more GAs for Metrico.