Is this reason for a reroll?

You need to send it, and then you can check later did he activate it by going to his profile.

beat his waifu till he accepts it. Or wait

Seeing as the giveaway ended less than 15 minutes ago, the winner probably hasn't opened steamgifts yet and thinks you're just some random guy. Try having a little bit more patience.

Is this a reason for reroll?

Yes. Up to you how you want to send your gift. If you try to contact the winner on email or steam and he ignores you then you simply wait the 7 days and ask for a reroll.

I've had a few people that never responded to the gift mail for various reasons. In all cases I've gone with a reroll

I suggest sending the winner an e-mail to notify him of the win. If he doesn`t respond after that you can most likely get a reroll

You better state your form of contact in your GA page for both parties to be OK beforehand...

A Steam user doesn't necessarily need to add random friend requests considering some users use different aliases here and on Steam. If he's not novice, he won't check if the request comes from GA creator checking his profile link from here...

So better write in GA description the way you'll send your gift...

Send an e-mail telling them they've won and to contact you through Steam to accept it. I recommend this to everyone. It's the safest thing to do. After a week, if they haven't claimed it, you can send in a re-roll request with a screenshot of the e-mail and we'll take care of it.

Edit: typo