Of course Gabriel isn't really dead; he learned long ago how to trick his way out of any conceivable deadly situation. But now he's in hiding, and what better form to take than a rambunctious kitten that won't stop following Sam and Dean around, interjects with a high-pitched meow and a strategically-placed claw when he doesn't like the way things are going, and likes to pee in Dean's shoes when he's not looking?