Steamgifts is completely legit. I got Titan Quest for free from there.
There is absolutely no way to get scammed. You enter giveaways for free. If you win, some guy sends you a game for nothing. If you win and he doesn't send you anything, you haven't lost anything except 10 seconds of your time clicking "enter giveaway". But 99% of the time the giveaway creators will give their gifts.
And no you don't need to give anything away to use the site. The "reward" for giving away games is increased chances of winning other giveaways because you can enter giveaways that can only be entered by people who have given away x number of dollars worth of games.