I been wondering that shit myself haven't got a straight answer, did a search and found this....
http://www.xbox360fanboy.com/2008/11/18/gears-of-war-2s-ranking-system-explained/
Gears of War 2's ranking system explained
Posted Nov 18th 2008 6:00PM by Dustin Burg
Filed under: News, Gears of War 2
After hearing about gamers' anger and general confusion surrounding Gears of War 2's online ranking system, team Epic has come to the rescue by explaining how the whole ranking thing works.
According to Gears community manager SixOkay, the Gears 2 ranking system strictly uses Microsoft's TrueSkill which is was created using chess' ELO rating system. This means that ranks aren't calculated strictly by points or kills, instead a rank is determined by your wins and losses AND whether or not you were favored to win the match. Also, the five different icons used to express your rank (viewable above) are used as a general range of ranks attainable, so small fluctuations in your win / loss ration won't instantly change your icon. Click the jump to read more about the Gears 2 ranking system and learn how it all works. Then, hopefully you'll be one less confused gamer.