From the Winter Sale announcement, my bolding.

"The Holiday Ninja Squirrel (have you met her yet?) brings a generous surprise for all with $3 Mystery Games from the GOG.com catalog, each worth between $9.99 and $34.99. These games won't be part of the regular sale lineup, and unowned games will be prioritized to minimize repeats. We're starting out with a pool of 50+ games, but we'll be adding more throughout the sale - we'll let you know when that happens."

I (fortunately) got Saints Row: The Third. I say fortunately because, when looking at the list, my chances of getting a game I don't want or one that I already have were pretty high.

I guess I was one of the lucky ones. And this is my first and last Mystery Game gamble, I prefer to quit while I'm ahead than risking getting something that's basically... well, crap, to me.

What you get purchasing the "Mistery Game" is a random $10+ game discounted to 66-75%. I'll better wait for such discount for games I really want then buy them.

I found a link in the smaller thread about this to the larger winter sale thread with a post by someone from gog(sorry don't have the link handy or quite remember the name) bu the posts indicates:
either it's fixed or in the process of being fixed and should be active by 'tomorrowish'...

I'll take that to mean buying another mystery game will result in a borked checkout for a few hours until it self clears...

[EDIT]
@vech32 well my problem is that I pretty much already own most of the games that I'm interested in or at least as far as the older titles OR I'm interested in some newer releases that I don't own yet and don't have good enough discounts to make me bite, i.e. really things that I'm not sure that I'd like, haven't finished the first game, etc.

ALL the stuff that I WOULD HAVE grabbed right away already is plastered with 'OWNED'...

Oh and there are a very few that I own on steam and wouldn't mind duping on GOG, BUT again discounts while good just aren't good enough for me in a case where I'm just duping as kind of an insurance policy...

...and then there are a few older games, e.g. the SSI AD&D games, and older Id FPS which I just think are whacked on their base prices... the Id games in particular are weird as Bethesda games are more reasonably priced and tend so far to have better discounts on top of that, so I nabbed all of those already...

The AD&D stuff, I have at LEAST 2 collections of IIRC ALL of the AD&D games that were sold in something like 20 CD sets uin the late 90s or early 00s budget collections IIRC... they're buried in the basement somewhere, so that's why I'm balking at the ridiculous prices for them on steam as those collections sold for c. $20 IIRC or that's what they were when I purchased them.
[/EDIT]

Well they've semi fixed the problem now as for future games. It'll tell you if it's a game you own before hand now . . . I sure hope they keep it this way. I have 0 interest in getting games I already own(even in gift form).

Huh, I'm also a little scared to try my luck after seeing the list so far. There are so many games that I already own elsewhere, and not a ton otherwise that I'd actually want to play...might be cheaper to just wait for a sale on the ones I want. I don't have much of a stomach for that kind of gamble.

That, to me, was a very, very fine game. Enjoy!

I have bought a mystery game, but I didn't receive any game, in order receipt history there is "mystery game". I also cannot buy another mystery game, because "I already own Mystery Game in my library"...

Yeah, I did wonder if that might be the case when I looked into it further.
Just got overexcited when I found something, should have done the further research first.. XD

@mrkgnao
That makes sense I suppose..

Known bug. According to GOG, should be fixed for future purchases. The pending mystery game should be converted to a real game by "tomorrow-ish", again according to GOG.

You got lucky (in that GOG is probably messing with the system and you made the purchase when it was working correctly). I bought three games and all were repeats even though I don't have everything on the list.

I just got Gurumin: A monstrous Adventure. I am super happy about it, because I would have never gotten it otherwise and it looks interesting to me.

Probably the "limited quantities" effect is kicking in on better titles now. I don't bother with these random grab gimmicks after the first hour of a sale, because typically all the good stuff runs out first.