While the gear box reduces the engine speed so it can keep the prop tip speed at a reasonable number it also multiples the torque out put of the engine.

As an example: if your engine generates 100 hp and 50 ftlbs of torque at 5800 rpm and you add a gear box with a 3:1 ratio then you now have 150 ftlbs or torque delivered to the prop. Horsepower doesn't change but the torque to drive the prop is multiplied by the gear ratio.