The Crimson Omen is an insignia used by the Coalition when an area has been overrun by Locust. It serves as a warning to turn back while you still can, and as the games logo as well fitting in with the desperation feeling of the game.
The insignia combines both the Coalition’s trademark “COG gear”, and a skull. To give it both meaning, and character. This symbol is always near a set of COG tags in the Gears of War campaign mode.