Gravity feeders work two ways for two different matters. If you’re looking at using a water dispenser, the gravity will work using pressure from inside the bottle, which is increased when the water bowl becomes empty. As soon as the bowl is full, the pressure of the water bottle holds steady, meaning no more comes out than is required.
For kibble and dry cat foods, the gravity-based pet food dispenser works by allowing the food to “block” the exit of the feeder, which can become “unblocked” when your cat eats the food that is in the way. Once this food has been moved, the dispenser will then allow gravity to it’s work, and the bowl is re-filled.