Yes, most certified fuel flow gages in small aircraft are really pressure instruments if they have analog (steam) indicators. You can usually see small markings in psi which are used in calibration.
The pressure reading comes from the fuel distribution block or spider. The theory is that the injectors are calibrated nozzles and by reading the pressure drop across them, the flow rate can be determined. The exact analog is using a shunt and measuring the voltage across it to derive current.
The down side of this approach is the nonlinear pressure vs flow ratio, leading to nonlinear gage markings and the need for the injectors to have a very accurate flow rate. Indeed a clogged injector can cause the flow indication to be inaccurately high and confusing since the cylinder with the restricted flow may have a high CHT/EGT indication.
So if you want to use the gage, make very sure it is used with the exact engine you have and you will probably want to have it calibrated.
Deene Ogden
RV8QB
EAA Tech Counselor/Flight Advisor