Silver members can enjoy a few features on Xbox Live, but multiplayer is not really one of them. That'll change this weekend though, if only briefly.
According to the official website, all Gears of War 2 owners will be able to play together online this weekend, regardless of whether they have Xbox Live Silver subscriptions, or the premium Gold memberships.

Epic is celebrating the release of the DLC pack Dark Corners, as well as All Fronts, both of which are now available on the Xbox Live Marketplace. Not only are they inviting in the Silver members, they're holding a Double XP weekend too.

Gears of War 2 owners can pick up Dark Corners for 1200 Microsoft Points ($15), and All Fronts for 1600 Microsoft Poitns ($20). In the meantime, enjoy the time you have online while you can, Silver members. The free weekend will run until 11:59 am EST on August 3.