Why don't small props have a gear?
Because there is no link between the wheels and the engine.

When the car is moving slowly, wheels turn slowly so the engine must turn slowly as well. However piston engines do not work below certain RPM, so the gear switching must be implemented even for small cars.

Propeller can always turn at the speed sufficient to sustain the engine running. If the plane moves too fast (or must stand in place), wheel brakes can be applied.