Vegetable fiber has no place, however, insect chitin is effectively fiber. Every time they feed on arthropod prey, they are getting their needs in that regard, assuming there is one.
fiber helps animals have enough bulk in their poop to move it easily through the digestive tract, tho it has no food value, being indigestable. when a gecko preys on insects, it also ingests the food that the prey has eaten, including vegetable fiber. whether this is 'necessary' is possibly debatable, whether it is natural isn't.