"Information: There are actually two unique errors that use this same text. The first error has a red “X” icon and the second version of the error has a yellow exclamation point ("!"). In a nutshell: for some reason Gears cannot find your DVD (or the completely wrong DVD is in the drive) or has found a DVD that doesn't look like it is the right Gears of War disc.
Resolution: This has most often been linked to an incompatibly between Gears of War and some virtual drive programs that could be running. A virtual drive program allows the user to use a portion of hard drive space to emulate a different drive or type of drive."