BMEP gauges are simply torquemeters. For whatever reason Pratt&Whitney decided to lable them as psi of Brake Mean Effective Pressure, an abstract theoretical quantity representing the average pressure in the cylinders during the power stroke
As an aside the BHP can be calculated from the formula BHP = BMEP(PSI)*Engine Displacement*RPM/792,000 As the quote cites, BMEP is not the actual pressure within the cylinder, but an effective pressure representing the mean gas load acting on the piston during the power stroke.