Won a duplicate code, GA creator might've blocked me

reply in the giveaway unless you're blacklisted. else mark as not received after 7 days. ^^

Actually, Steam doesn't give any response if the individual has you blocked, so you're working off a mistaken premise.

Googling the matter, it seems there's a well-known bug; the recommended fix is to block, then unblock the user, after which friend adding should resume working. The other most commonly recommended fix is making sure your account isn't set to limited status (due to not having made a direct purchase from Steam for a year). Other suggested fixes include verifying your e-mail address, switching to another computer so as to establish a fresh login session, and trying from there, or- and this was stated to be the official Valve recommendation- typing steam://flushconfig into your browser to reset your core Steam files. In short, the issue is indicated to be something on your account's end, and is not anything which involves the GA creator in any way.

While bugs should preempt the GA creator getting any notice of your attempts to contact them, do note that blind-adding someone on Steam is generally considered quite rude. As such, if you didn't leave a message on their giveaway indicating you were going to attempt to contact them on Steam, even if they got your friend invite they may still be intentionally ignoring the friend request (especially so if they can't figure out why you would want to be adding them)- or, like me, they may simply never ever look at their invites page unless someone indicates they should do so.

Likewise, the first place staff looks for information on a giveaway is in that giveaway's comments (meaning that's by far the best place to leave information), and staff has no way to force the GA creator to give you a game. Typically, the only times a winner should involve support (that come to my mind) are if they believe some sort of exploitation of site rules is occurring, or if the GA creator has blacklisted them, making direct communication impossible, and the winner wants to update staff on the situation.

In short, when winning a giveaway, and having an issue:
1. Contact the GA creator in their giveaway to update them and staff on the situation.
2a. If the GA creator has indicated you may do so in the GA description, or if seven days have passed you may contact the GA creator on Steam.
2b. If the GA creator has blocked you, you may contact SG staff to update them on the situation, and have them contact and update the winner on your behalf.
3. Wait patiently for seven days to pass, after which point you can mark Not Received and may resume entering new giveaways for the game.

Past that framework, there's little you can do (and certainly, not much that's advisable).