The survivors of Cannae numbered enough to form two legions. They were forced to serve constantly during the remainder of the war, never being allowed to return to Rome. Scipio and Gaius are destined to be the ones to slowly grind down the Carthaginians until they force Hannibal to finally face them in Africa.
So, yeah, Gaius's reprieve from war is going to be very short-lived, unless his story takes an unusual twist, like becoming a bandit or something.