Gravity will siphon water out when the system is not fully sealed (priming opens a valve to allow air to escape.) Once the system is closed gravity plays no part because there is just as much gravity pushing down on the intake side as there is gravity pushing down on the outflow side. Test it with your cannister, with the intake and outflow both submerged place the cannister on the floor and measure the flow rate, then elevate the cannsiter and measure again, you will see no difference in flow rate, gravity is not a factor. Imagine a 6" piece of tubing submerged in your aquarium vertically, water won't flow from the top of the tube to the bottom of the tube because of gravity, you'd have to physically pump water into the tube to create a current, gravity won't do anything in this case. By the same token you couldn't take a piece of tubing say 24" long with one end near the bottom of the aquarium, start a shiphon into a bucket then plug the end of the tube and place it toward the top of the aquarium and expect it to continue to siphon water and circulate it in the tank, you'd need a pump to get water movement in a closed system.
I won't continue to argue the point, it's something you either understand or you don't. Gravity is irrelevant in a closed system, end of story, get a physics book if you want further clarification or do a google search for hydrodynamics or hydrostatics. A sump is not a closed system, your thought process applies to sumps if that makes you feel any better.