Why can gear pumps pump very viscous liquids without check valves?

Gear pumps do not require check valves because they operate as rotary positive displacement pumps that use the meshing of gear teeth to move fluid.

In a gear pump, fluid is drawn into the pump by a partial vacuum created as the gears unmesh. The fluid is then trapped between the rotating gear teeth and the fixed casing, which carries the liquid to the discharge side. As the gears mesh together again, they generate the pressure necessary to force the fluid into the outlet line. Because the gears themselves act as a continuous seal against the casing and each other, they effectively prevent backflow, allowing the pump to handle very viscous liquids without the need for external check valves.1