A siphon gun has a container that hangs under the body of the paint sprayer. They lift the paint out of the container using a tube. These types of guns require higher amounts of air pressure than many of the other guns. They do not cover a large area when they are painting. You will likely spend twice as much time painting if you decide to use a siphon gun.
A gravity filled gun requires a smaller amount of air pressure, so you can have a smaller air compressor. The gravity fill gun works great in small areas. The paint container is on the top of the gun and gravity allows the paint to enter into the necessary chamber of the gun. The problem with the gravity filled guns is that you will spend a large amount of time refilling the container cup.