Oil pressure gauge on these trucks is random, that '88 should still have the real gauge with a real sender as opposed to just a switch that makes it read normal anywhere between 8 psi and 80 psi, but still they can be quite moody sometimes. Ours drops to nearly nothing from time to time, mechanical gauge hooked up to the same port shows now issues, sender's been replaced twice, so it's gotta be the gauge.
On the question about grounding the fuel gauge, no it does not have its own ground, instead it shares one with the temperature gauge and the electronic module that turns on the red "engine" light when either the oil pressure drops too low or the coolant temp gets too high. It also lacks its own power supply, it shares the 12V key-on again with the temp gauge and the warning module but also with the oil pressure gauge and the voltmeter. By the way the oil pressure gauge and the voltmeter ground together via separate ground wire, they share that one with the tach also. Why not ground everything together idk, but knowing the shared powers and grounds makes it easier to troubleshoot the gauges - for example if fuel and coolant temp are both acting up suspect their ground, if oil pressure and voltmeter are acting up suspect their ground, and if they're all going bananas then suspect the key-on power.
In any case it's possible the gauge seized up, wouldn't be a very common issue but these things are getting old so anything is possible. The cluster is of modular construction, the small gauges come out in vertical pairs and the speedometer and tachometer are their own units as well, so you don't need a whole new cluster - just hit a U-Pull-It yard for the block that contains the fuel and whatever gauge is right above it, it will drop right into your cluster.