You get Gears of War for ~£7.64 (10$) in the Xbox Store (US). You need a Gold membership.
Experience the game that launched one of the most celebrated sagas in entertainment with Gears of War: Ultimate Edition. The shooter that defined the first generation of HD gaming has been painstakingly remastered in 1080P and modernized for Xbox One, and is packed full of new content including five campaign chapters never released on Xbox.
The story of “Gears of War” thrusts gamers into a deep and harrowing battle for survival against the Locust Horde, a nightmarish race of creatures that surface from the depths of the planet. Players live and breathe the role of Marcus Fenix. A disgraced former war hero, Marcus seeks personal redemption as he leads his fire team against an onslaught of merciless warriors from below.