The expiration date on the certificate was apparently set for yesterday, January 28. When the date switched over, the game became unplayable.
"This was a surprise to us too," said Epic programmer Joe Graf on the company's official forum. "We have been notified of the issue and are working with Microsoft to get it resolved."
A similar issue recently disabled Turbine's Lord of the Rings Online. As in that situation, users have found a simple workaround for the Gears problem: setting the system clock back to a date prior to the expiration.