AndreyT is 100% correct: as long as the inlet and outlet are underwater in the tank, gravity has no effect on the operation of a primed pump. In that case, the pump is only working to overcome the resistance of the hoses, which will be greater the longer the hoses. Now, if you raised the opening of the outlet hose above the surface of the water, all bets are off: now the pump does additional work to return the water against gravity. I've seen this on plenty of occasions with a small pump trying to get water up to a higher tank. If the pump is only strong enough to make it up to the tank but not over the lip, you get no net flow. But if you run the hose up over the lip, under the water, and suck out the air…bingo=flow. Not everyone has the equipment to measure the output of a pump, but anyone with a small powerhead and a bucket can demonstrate this reality for themselves.