hummer010: Apparently I don't! I've tried to give it away via chat several times, only to have the recipient tell me they had already received it. Apparently someone finally took mine, because the key no longer shows up on my unredeemed gifts list.
I'm sure I'm not the only one with a spare key of it lying around. I believe that Torchlight holds the dubious distinction as being the most given away game by GOG outside of the free games.
MiloRivers: How often does GOG have these giveaways? (Another reason Steam can suck a fart)
Usually, every major sale gets one giveaway.
The last winter sale ended with a 48-hour giveaway of Giants: Citizen Kabuto, the last summer sale had one for Battle Realms. Additionally, you could "earn" free games by spending a certain amount of money ($5 total got you Divine Divinty, for example).
Occasionally, they'll also do giveaways in the course of some other "event". A "Team 17" promo on December 14 came with free copies of Worms Forts: Under Siege.
There are exceptions to this, of course, but by now, it is almost tradtion here.