Per the manufacturer, yes, the Hypro 6500 C-R Roller Pump will work with a 6 HP motor with gear reduce as the gear reduction is a 6 to 1 (6:1) gear reduction.
Per the manufacturer, whatever engine you are using with the Hypro 6500 C-R Roller Pump would have have to have a 6 to 1 gear reduction on it. Without a gear reduction on the engine, it will not work.