Maybe a fuel pressure gauge is useful in a jet test aircraft but not so much (usually) as either you have it or you do not. There is a filter bypass light in addition to any other instrumentation. If that comes on then one can be in an unusual situation. One dark and (not stormy) night returning to Anchorage in a C-130 (Ok, L382G) the fuel filter bypass lights started winking. Made them go out by turning on all the "override" (higher pressure) boost pumps. On approach the plane felt minorly weird, #4 spools down as I try to bring all four into reverse... Plane had been in Africa where it had picked up a bunch of lint in the fuel...
Maybe useful on start as a crosscheck on an early engine. Proper fuel pressure is necessary for a good spray pattern to develop, no good spray pattern, hot start! The engines eventually had a pressure relief valve that would not allow fuel to the nozzles till a minimum value was exceeded. Not sure about the "early bird" engines.
T