Probably not a sequel. Go here http://en.wikipedia.org/wiki/Gears_of_War_(series)#Other_games. It is likely the next Gears of War installment shall be a prequel
In an interview by IGN with Joshua Ortega (Gears of War 2's writer), Ortega responded to a question regarding a Gears of War prequel dealing with Emergence Day saying, "You will not be disappointed in the next ten years; it's a ten-year plan. Gears is long-term. The Lancer is the new Lightsaber." Even with Ortega's statements, a prequel game or any other games in the Gears of War franchise have not been officially confirmed to be in development.