The unit of measurement is always mechanical torque i.e. a known force applied at a known radius from the axis of the coupling shaft. Typically the unit is a pound-foot (*) or perhaps in EU nations, a newton-meter.
That said, the cockpit may be simplified by calling the engine (or gearbox) spec value 100%, and scaling the instrument appropriately. Makes for easier type-to-type transition.
The same scaling occurs in RPM instruments on turbine equipment. Rather than specifying max power as 17,876 rpm, that value gets labelled 100% and the gage calibrated from there.
* The unit of physical work is a foot-pound per Watt's definition. (Note the labels are swapped)