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You can test the GPCM and glow plugs with an ohm meter, but you don't need to unless your check engine light bulb is burned out. If your truck has a GPCM (all excursions do), then it will throw a code and trip the check engine light if you've got a bad glow plug or GPCM.
That means you don't have a glow plug issue. (unless the previous owner installed autolite glow plugs which might turn into a big issue down the road). Might be worn injectors. What's your comfort level in pulling the valve covers to take a look see?
What would I be looking for taking the valve covers off
Oil discharge coming out of the spout at 11:00 on each injector while cranking or when it finally starts.
While cranking or when running, each injector should be spitting about the same amount of oil as the others. If you've got several injectors not spitting oil on a cold start, but they do start to spit oil after the engine warms up or has been plugged in for a couple of hours, then that indicates worn internals on the injectors.
They're not clogged. You don't need some magic injector flush to clean them. The injectors on the 7.3 use engine oil to hydraulically actuate the internals and ramp up the pressure of the diesel fuel for injection. If the injector isn't spitting any oil, it's not injecting any fuel. The issue here is on the oil side of the injector, not the fuel side.