You will be disappointed to hear that there is to be no Gears of War demo out before the game's 'emergence day' on the 17th November
Regarding the rumour of a pre-release demo, Rein posted on the official GoW forum, "It just isn't true. There is no demo coming out before the game ships and I have no idea if we'll do one after the game ships or not. Right now there are no plans to do one…"