It seems that you're using an outdated browser. Some things may not work as they should (or don't work at all).
We suggest you upgrade newer and better browser like: Chrome, Firefox, Internet Explorer or Opera

Ok, here's the thing. There are two games I don't have in the Mystery Game, and yet I got a repeated game (no gifting involved). I might have found why. When I go to my orders, I have the one I paid, and two unpaid ones for the same games. The first time I got to checkout to remove titan souls and add titan souls: digital special edition (it's also on sale, though not in the front page). And they I didn't pay from my computer for safety reasons... so I got this two other orders linked to me -with a timer- with a mystery game attached to them. Maybe for the games I don't have (...)
Mysteries, oh, the mystery!
Another one to add to the bug list.
I'm hoping for Anne's Quest to be revealed. ;)
Bard's Tale, The
Airline Tycoon Deluxe
Gray Matter
Crusader Kings Complete
HunchBluntley: As with any other purchase, make sure the check box that says "Make this a gift order" (or whatever) is checked on the checkout page before you go through with the payment.
But remember, buying as a gift will pull the game from the whole pool, rather than just what you don't own.

GOG has already converted the unwanted games from my Mystery Game purchases into gifts, but as the sale goes on I expect their backlog of such requests will increase.
Lilly Looking Through
Consortium: Master Edition
Breach & Clear
i got Space Colony HD and a Mystery Game
IAmSinistar: Be warned, if you buy the Mystery Game as a gift, there is no guarantee that you won't get a game you already own. This has been standard practice in GOG sales for a while now. Basically the computer picks the game at time of purchase, and when you buy a gift code, it assumes that anything goes.
More people would probably see this if it was in the OP as well. : )

Also, I didn't see Mystery Game in your list -- that's one of the top games people are reporting they've gotten! But I'm sure this was an oversight. : P
Also also, there's no 'key' in the title of Spelunky, just within the game itself. ; )
Told myself I wouldn't do these gambling promos anymore after spectacularly bad luck the previous times, but I couldn't resist trying one. King's Bounty: Warriors of the North - Complete Edition, oh well.
HunchBluntley: More people would probably see this if it was in the OP as well. : )
Good point. I'll add it as a PSA. Because knowing is half the battle! :)
Man, so many awesome games, but the pool is to big. :P
Bought it as a giftcode and i got:

Cursader Stronghold HD

i already owned this game, but its a decent mystery gift
I got Long Live the Queen
NuffCatnip: Man, so many awesome games, but the pool is to big. :P
I find it too small. None of those games are on my wishlist.
Fairfox: I want to do this, but I ate total shit last time.

I will not do this.
Did you do it? :P