Can I enter a giveaway for a friend?

You should only enter to win giveaways for yourself, and if you win a game, it must be activated and redeemed to the Steam account used during registration. If a friend is interesting in joining giveaways, they'll need to register for their own account.

The part about 'must be activated and redeemed to the Steam acount used during registration' should make it clear that you can't give the game that you won here to anybody else. Re-gifting is just adding insult to injury...