It's not really difficult to trace the development of shooter games to certain landmark titles. Once Half-Life got big, you started to see the inspiration of its story line and level design trickle into other games on the market. Once Halo hit, the idea of a regenerating health bar instead of the Doom-style medpaks took over, and Gears of War popularized cover movement and squad tactics that had normally been the dominion of super-realistic titles like Rainbow Six. Damnation is pretty clearly a post-Gears of War title, but it isn't really fair to call it a knockoff, because while it's heavily inspired by Gears of War, it's actually trying something quite unique. According to Codemasters, Damnation isn't just a shooter. It's a vertical shooter.
Damnation is set in an alternate timeline. Due to unexplained events, the American Civil War never ended. Instead, it continued to escalate, sparked on by military-industrial robber barons, who continued to pump out new technology in an attempt to maintain their stranglehold on the wealth of the country. The worst of these was a robber baron named Prescott, who by the 1900s, has turned America into his own private fiefdom, complete with an army of mercenary soldiers equipped with the finest weapons to roll off the Prescott assembly lines. You're put into the shoes of Rourke, a skilled gunman and leader of a group of rebels who are seeking to stop Prescott and put an end to his reign of terror once and for all. It's no "Casablanca," but the alt-history Civil War setting adds a neat visual flair and allows for a bit of freedom compared to the usual dank city or space station locales that most shooters choose to populate, which is quite important due to the vertical shooter concept.