Pumps usually use GPM. Maybe you see a large number, 100 GPH, thinking it's a high capacity. A March pump is 300 GPH (5 GPM) at 7 ft. head. Maybe your application requires less than 1.6 GPM. Depending on what you're trying to do. You may be able to vary the volume with a bypass valve, flow valve and associated piping arrangement, if you are requiring a trickle.