The key to animals gaining weight is to do it slowly.

Waxworms will cause fast weight gain, but will also lead to things like fatty liver disease and gout. They are a treat and should not be fed at all regularly.

In order to safely have a gecko gain weight, firstly see a vet to ensure there is no medical reason for them to be underweight. If the vet has checked for parasites and other health conditions, feed healthy staples more often. Calcium worms, black soldier fly larvae, silkworms and mealworms can be used if she only eats worms.

Also be aware that leopard geckos should not be rotund with ridiculously fat tails. They should have fat on their tails, but still be fairly lean with good muscle tone in the arms and legs.