In a quick "better late than never" update for you, the two games that Microsoft is making available for free the month of December for Xbox Live Gold for Xbox 360 subscribers are Gears of War and Shoot Many Robots. As always, they are yours to keep and play forever.
For those who may not be familiar with the games, Gears of War is a military, science fiction third-person game developed by Epic Games. It was originally released as an exclusive title for the Xbox 360 back in 2006, and has players assuming the role of Marcus Fenix (or Dominic Santiago) and focuses on a last ditch attempt to same the remaining human inhabitants of the planet Sera from the Locust Horde. This game is available for free to Gold subscribers until December 15th.
Shoot Many Robots is an action platformer that has players fighting off robots by using weapons, melee attacks or other special attacks as they work their way through the game's levels. It also supports cooperative play between two and four players. This game will become available to Gold subscribers on December 16th until the end of the month.