As the picture suggests…

Go Gold between April 9 and 25, 2008, and get it all:

1. Connect and play with the friends you have and the friends you’ll make.
2. Shoot ‘em up in Heavy Weapon: Atomic Tank with up to three of your friends—and enjoy the whole range of Xbox 360® multiplayer games!
3. Enjoy both exclusive and early access to Xbox LIVE Marketplace content.

To get your game, follow these two steps:

1. Go Gold by upgrading your Xbox LIVE subscription from Silver to Gold 12-month membership.
2. Then register here to get your game.

Offer ends April 25, 2008

Via: Xbox Newsletter