Desura Games?

You can give it away in the forums, but you can't make an official giveaway for it

Not as an actual giveaway, though you can give it away through some other means on the forum. Only Steam games for creating giveaways here

On the forum, yes. As a SteamGift, no.

Nope, but make sure you don't actually type out the game key. People make scripts that scavenge the forums for keys and take them within a few seconds. Try putting the key in a picture or something.

Ok, thanks guys!

its possible but only in the forums as it seems.