I've unlocked SPELUNKY and GABRIEL KNIGHT. Unfortunately I already own GK (Tremendously good game). I'd like to give away my copy to a friend. Is that possible?

If you already own the same game here, then clicking "Redeem", will send a code to your e-mail. Give it to your friend and he should be good to go.
If, however, you don't have the game here, then clicking "Redeem" will just add it to your library and you can't convert it to a code.

No way to get a gift code even if you don't own the game huh? Not really interested in Splunky.

Nope, no way. Either you get one by already having the game here before you click redeem or it gets automatically added to your library, with no way to convert it to a giftcode (support has refused to do so).