Epic explained: "The online cheat detection features in Gears of War for Windows are based on digital signatures. Well, we made an embarrassing mistake: we signed the executable with a certificate that expired in a way that broke the game.
We're working with Microsoft to re-sign the binaries properly, and hope to have this fixed very soon. We know how much this situation sucks, and we apologize for the inconvenience.
In the mean time a work-around for this is to set your computer's date back to a date before today."