Your fuel reg is upside down, the actual weight of the fuel inside will be acting on the 2 diaphragms inside that seperate the fuel air(vac side), and pushing against the vac side.
It needs to be vac pipe at the top, along with adjuster screw, fuel feed in the side(as it is) and fuel return on the underside.
As your gauge is not going to have two fuel returns from it, the pipe where you connected the gauge must be another rail feed, it's not a 2wd(cossie) pressure reg and doesn't look to be a 4wd one either, the 2wd reg has two threaded holes on the underside either side of the return pipe that the bracket bolts to, and the other end of the bracket secures on the top corner rear bolt sticking through the plenum, with a 10mm nut.
That being the case and the location of the extra pipe it is in the fuel side so will exhibit precisely the same pressure as the rail before it, you can show this by squeazing the return hose briefly whereupon the gauge reading will go up, due to the "blockage" of returning fuel, it doesn't matter which side you measure it with this type of fuel reg, you could simply unplug and turn around so it is reversed if you wished, makes no difference, the pintle(valve) inside which controls the pressure is in the area immediately above the return pipe(inside), connected to the two thin diaphragms inside, so that the pressure changes with vac or boost as needed.
tabetha