link to generate the key not the key directly

you will see winner email the moment he/she is chosen ;)

Once your giveaway ends, the winner's e-mail will be shown here.
Personally, I prefer adding the user on Steam and then sending them link through the Steam chat.

You can manually email them (with your own email) the gift link and have them input their email address -
I've also had one giveaway creator input my email address for me, which would then trigger Humble to send me an email. The downside of this is the gift link gets used up and can't be sent to another person if the game ends up not getting claimed.
Of course, the email they put into Steam Gifts should be one they will use to claim the game - which should show up when you view the winners of your giveaway.
Make sure you take screenshots in case anything goes south. This rarely happens, but it can still happen.)

Edit: Too slow. Few others beat me to it.