Well, I'd think that if you were working on a transmission, the pressure gauge to use would be a transmission pressure gauge. You would have to find a pressure port on the transmission where you connect a pressure gauge that would allow you to measure the pressure. Ever heard of an oil pressure gauge? When your engine is running the oil pump pressurizes the oil galleys which is the pathway to the bearings and such. In a transmission, when you select a gear, a series of valves directs fluid to apply a clutch via a piston of various designs. This pressure can be measured by removing a plug in the side of the transmission, installing a gauge capable of measuring up to 300 psi. You start the vehicle and see what pressure is applying to the reverse input clutch. Approximate pressure would be over 250psi. You could adapt your compression tester is you don't mind destroying it with filling it with oil.