Input power can be anything from enough HP to turn the input shaft to trillions of HP.
Your information given is insufficient for a proper answer, because you evidently do not understand what Power is.
Advise output Power required, and the input power would be around 120% of the gearbox output power, for the range of 1HP to 10HP approximately, dependent on gearbox lubrication and temperature.
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What material; mild steel, toughened alloy steels, case hardened, cast iron, SG iron, brass, bronze? Material properties and manufacturing methods? Hardness of core and tooth surface? Tooth width? Diametrical Pitch, ie teeth per inch? Is the contact bevel with intersecting teeth or hypoid with shafts offset etc?
Well thats a start. There are different calculation formula for strength and wear and you have to take the lower HP result of the two for safety and continuous use. You can make a rough calculation if you have some of these values and guess others, but I dont think you will obtain an accurate figure.