From Elizabeth's care sheet:
In emergency situations during which a leo has been off food for extended periods of time and is losing weight, REHYDRATE FIRST for a couple days!!! A few drops of unflavored Pedialyte on the gecko's snout or in the water could help jumpstart the appetite. Do not attempt feeding a dehydrated gecko till the gecko's hydration status has been confirmed by a vet.
Rehydration Information
To entice any gecko to open his mouth brush a thin dowel, a stainless steel gavage needle (with a rounded tip), or your finger horizontally past his lips. Stroke gecko's lips, gecko opens mouth, then insert water, food, or medicine. Be gentle and never force the mouth open.
How to force feed a Leopard Gecko - YouTube
Recovery Foods
***Fluker's Repta+Boost -- good for leos off food
***Oxbow Carnivore Care (premium recovery food) is even better. Available through a vet -- mainly. Oxbow Animal Health | Carnivore Care
***Lafeber's Emeraid for Carnivors -- quite similar to Oxbow's Carnivore Care
***In a pinch, use Mazuri's Mazuri Shopping Cart - Insectivorous Lizards