In addition, once you “purchase” the free game you’ll get a 20% off voucher code good for anything in the store in your confirmation email.
(Update: Free game offer is between 1100 UTC July 11th until 1100 UTC July 12th 2012)
The game will not activate on Steam. You’ll have to use the GMG capsule program to download it.
(Edit: BTW, the reason it’s a Winter Sale is because there’s a bunch of countries in the Southern Hemisphere that are currently in the Winter Season.)