I don't think there is anywhere left for Zarek to go so I'm guessing he dies in the next episode. But how? One plausible scenario is that Zarek is on the verge of doing something horrible, possibly threatening to nuke a civilian ship, and Gaeta decides to stop him by killing him. I don't know if Gaeta can survive this either, he'll certainly never serve as an officer on Galactica again, but he might try to redeem himself in some way before he goes.