What should I do?

you mean he Couldn't claim it..talk to the winner..give him a copy that works in UK & take region restricted copy back...
if he doesn't respond in 7 days then contact support

How or Why did he enter a region restricted GA? :/

Claim the gift back if he didn't accept it and ask for a reroll.

if i get this right you made a giveaway for RU+CIS which is avaliable for the users with the ip from those regions. it doesn't matter what it says on winner's profile. either the winner is from one of those countries or it is a bug. contact the winner. if they're indeed from the uk, take your gift back, tell them to submit a support ticket to fix their region and reroll. if they're not from the uk, have a little chit-chat about why did they write they're from the uk on their profile, which bands are trending currently, which tv shows the uk people likes, why there is a stereotype about english people's teeth etc.

wait... the end of this didn't make sense. i guess i should have written it when i had a clear mind. .-.

If the rules are crearly written that the gift is only for RU/CIS and he still entered take a new winner like.
He doesnt deserve it then and someone from Ru/CIS who entered cant even get it.

Has it been a week already? A winner doesn't have to claim the game right away.

So, now I cannot add him, steam and even browser says "Error adding friend. Please try again."

He is a russian. That is a common practice for russians to fake their info. Just reroll.

he finally took it, but added to his inventory. So I guess I should just add him to blacklist and forget about it.