This was part of today's announcement.
Source: http://news.xbox.com/2014/01/games-microsoft-studios-gears-of-warXbox Wire said:Phil Spencer: It all comes back to our commitment to Xbox fans. The Gears of War franchise has a very strong, passionate and valued fan base on Xbox. Over twenty-two million units have been sold across all Gears of War titles worldwide, grossing over $1B dollars (US).