What's the general procedure/etiquette for unclaimed GA's

Have you tried to contact him/her via steam?

You may not be sure he already used the code (on another account) so don't reroll after 7 days if he came online after winning your giveaway.
If the winner don't have a private profile you should contact him with a comment on his profile or even on one of his artwork/screnshot (if he got any).

I made a request to add him as a friend - still Pending.

Looks like comments are disable for non-friends :(

Give them 7 days. Just because somebody has logged into Steam it may have just been over an internet browser and they weren't in a position to activate keys. I have a winner who is on holiday and in that position and has asked for patience and I'm fine with that.

I wouldn't go 4 days myself without at least contacting the gifter, but different people here have different standards, Give it a few more days and try not to worry.

Curious about this too if it were to happen. I've read the FAQ's but have more questions if it ever happens. I've been very lucky so far !
FAQ says "please contact support to request a new winner." But after 7 days, the person giving away the game may not want to re-roll because the key has potentially been used... and if the giver can't afford to buy a new back up copy...they may not want to risk the re-roll...
I'm guessing it would filed as support - (Category) -User Support (Reason) Not activating won gift. Is what the user names in red on won gifts indicate? Does the giver still get CV? And do the winners still have the opportunity to activate their gifts and mark as received when they finally return? or should the GA creator immediately remove the key? Does support automatically just set up a re-roll for the user not activating support request?

Give them 7 days, and then if not, then make a reroll support ticket. You'll need to provide some type of proof that you tried contacting them (email, friend request, pending gift, or a censored key). And then they will reroll it pretty fast.

Try commenting in one of their giveaways, so they'll receive a notification. That's what I usually do in this situation.

I think you'll have to make a support ticket after 7 days and give them proof you sent the game to the winner, after which they can mark it received for you.