Really, it's worth a look. My first thought of 'gravity pump' was some joker with a funnel and a hose, but upon starting the YouTube video it really was a pump. Having a background in hydraulics and pneumatics, the pump they are showing should work quite well. I love simple machines like this, only a few moving parts and it would be quite easy to build on your own.
It is somewhat like a ram pump in that it needs a source of water higher than the pump. The water only needs to be slightly higher than the unit though. (Total drop of 20 inches needed and the beginning of the video states they have lifted water to 40 and 50 feet.) The amount of lift provided should be constant, and the rate of flow dependent on the flow of the stream.
It is basically a see-saw where water from upstream is continually pushing down one side, and then the other. The up and down motion acts upon a cylinder beneath each side that is sucking in water from under the unit and pumping it.
You have to fill a PVC tube with water, once the tube is full it tips and provides the force to pump water. (the force acting upon the cylinder that actually pumps the water) It won't tip until full, so each stroke should have the same amount of force (weight of the water pushing down on the cylinder) to lift water higher than the pump. The rate of flow of the pump is dependent on the rate of flow in the stream. The quicker you can fill the pvc tube with water, the quicker the tube tips and the pump will cycle. Once it does tip, you'll see in the picture the reservoir where the water first enters the unit has a solid partition in it. When one side fills and tips, that partition is moved enough by the tipping action so that the incoming water is diverted to the opposite side filling the opposite tube.
It might not be the best choice for a lot of things but might fill a niche where other options wouldn't work.
Pictures with description of function
Video of assembly and operation
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