on bigger aircraft the value is often presented in percent since the raw number might get too high to write it usefully down on the instrument.
the interest for a pilot is a gauge which shows him the limits not too overstress the engine , not the factual number what it is.
no gauge in output of horsepower since only chasing a certain power output may result in bursting limits of temperature , torque or core speed in different ambient conditions or prop rpm settings.
cheers