Most pressure washers need the back pressure from city water in order to operate properly- including the pump on this pressure washer.
Success using gravity to feed a pressure washer depends on the situation- large trailer setups may have enough pressure, or can use small water pumps to make sure the pressure washer has enough water flowing to it.
Pumps that self-prime to use standing water can be found on Karcher, Kranzle, and Dirt Killer units.
Please make sure your water supply hose has a filter on the end to keep grit out of your pump, and be aware that gravity and hose length play a part in how far away from the still water source your pressure washer can be- usually it's not a very long distance.