Antibiotics take time to take effect. Some can also put reptiles off their food. And if yours is heading into another shed, that can also be an appetite deterrent.

Did the vet have any theories as to what could be wrong? What would have caused this? Did he give some recommendations for hand-feeding? If a gecko is not eating and losing weight, many vets will prescribe some Critical Care formula to be administered orally. It's designed to be an appetite stimulant.