Gear pumps are the most common type of hydraulic pump, and also the most widely used rotary pumps. Hydraulic pumps like gear pumps operate by trapping a fixed amount of fluid and then moving it into the discharge pipe. Gear pumps work specifically by the meshing and separating of gears. The gears separate from each other at the inlet pipe, causing suction, and mesh near the outlet, forcing the fluid into the discharge pipe. Hydraulic gear pumps are most often used for pumping oil, but can pump a wide variety of other fluids with ease. Most gear pumps use the fluid they are pumping to self-lubricate, so you will never have to manually lubricate your gear pump.
GM Pumps carries two hydraulic gear pumps by the Japan-based pump manufacturer Koshin LTD, the GL series, and the GB/GC series, so you will be sure to find a gear pump to suit your needs!