Either put gerbils out of the cat's reach or get rid of the cats or the gerbils. Cats are hunters. Gerbils are prey to cats. You can't punish a cat AFTER anything he has done. Like dogs and other animals, they do not understand past, present and future. They just are, in the now. You can't say to a cat "You ate that gerbil half an hour ago, you're bad, so I'm going to punish you".