How long was he on rats before his fast?

If for some reason he was on a fast, if he had taken many rats before, you at least know that the reason he went off feed didn't have anything to do with the type of prey being offered. Sounds like the gerbil just reawakened his feeding response. I would definitly try to offer a rat again sooner than later, while the whole idea of eating is fresh.

And since gerbils are very close to their natural wild prey, I would feed gerbils if I could afford to. If you don't mind spending the extra money, I'd say go for it.