How do you help a gerbil recover from food deprivation?

You don't have to do anything. Fresh fruit and vegetables arnt available in the dessert where gerbils originate from so it is no a necessary requirement in their diet. If you want them to eat fresh foods I would look up all the foods they can eat and try them on different things there must be something they like. Also if they weren't feed fresh foods by the first owner they won't be used to it or want it. My gerbils don't like fresh foods they just leave them. I tried them on different things. I found out one of them likes dandelion leaves but they all love stale, whole meal bread so you could try them. Gerbils can also eat lettuce, Apple, various berries ( look up which are OK, carrots, bananas as well as other things.