So what happens when someone who wins your GA doesn't confirm they received it??

have they activated the game on their account?

Well, I would suggest a truly miraculous method that very few SteamGifts users tend to consider:

Talk to the winner and ask them.

Contact support. If they viewed the key, "Request received feedback" might be your best option. As for repercussions, no I don't think there are any aside from not getting CV or the giveaway slot back until they confirm.
(Please correct me if I'm wrong.)

edit: What talgaby said. Laughing at myself for not thinking of that.

1) Reach out to the winner. Steam and email, if available.
2) Make sure the game has not been activated.
3) Pull back the key (if it's a key) then contact support to request a re-roll. Provide screenshots demonstrating you reached out to the user.

Guess it took me too long to type all of this out :P - Simon's answer is basically the same as mine. Anyway, thought you'd appreciate a more detailed step-by-step since you're still pretty new to SG and all :)

Maybe the person forgot to mark it as received - this do happen from time to time. You could start by checking the winner's Steam library for the game to see whether the key has been activated or not, and if it's in the library, contact the winner and ask the person politely to mark the game as received.

There are two main ways+1 extra way you can do so:

1. Go to the winner's SteamGifts profile and then click on their Steam profile. You can then post a comment on their Steam profile and/or send the person a friend request just so you can privately message him/her.
2. Go to the winner's SteamGifts profile and look for an Open giveaway (not Group or Invite-Only) that's either ongoing or ended, and then send him a message there. The winner will then get a notification on SG and hopefully, the person will then respond.
3. Email works too if it's available.

Of course, it's best if you try all three methods.

If all else fails (Give the winner a bit of time; you can actually see the last login of the winner on his/her SG profile), contact support like Droj said and ask for a "Request received feedback".

If the key hasn't been used, remove the key and ask support for a reroll. However, it's also possible that the winner has given the key away/sold it since the winner has already viewed the key so it's very risky to use the same key again.

Not on their steam account. They have been offline for 9 days on steam and have not responded to any emails. Funny thing is they are online almost daily on steamgifts!

This happened recently to me, a winner has viewed the key, activated it and didn't mark. I wrote him my usual standard e-mail and a message on Steam without any answer, so I opened a support ticket requiring winner feedback and, since the support staff saw that the game had been activated on the winner's account, they gave me the received feedback.

The first things you must do are: contacting the winner both on Steam (if available) and on his e-mail shown in the winners' page. you can also contact them on one of their giveaways, if you can enter them (ie. not group/wl only etc)
Secondly, if he does not answer, after 7 days you have two choices:

• if the winner has not yet view the key (the eye green icon) you just have to file a support ticket requiring a new winner, adding before of course the image (key hidden through MS Paint or whatever) proving that you sent the key to the winner and in 7+ days he didn't even view it
• if the winner has viewed the key, most likely he has activated it on his Steam profile and so you have to file a support ticket requiring winner received feedback. The staff members will take care of everything!
• if the winner has seen the key and the game is not in his Steam library, unluckily, you have to file a support ticket asking to delete the giveaway since you can't be sure that that already-viewed key is still unused and brand-new

on the third option, if there is the revealed icon and so you saw that the winner HAS viewed the key, BUT on his Steam profile the game is not activated.. he probably has kept it, given it to someone else or whatever.. I wouldn't risk at all to use again that key.. so I'd request a delete giveaway..