How do you i test the oil pressure gauge on a 1998 Chevy?

Take the wire loose from the oil sending unit and then turn the key on and ground the wire to the engine somewhere and the gauge in the truck should go all the way past the highest point. If it don't then the gauge in the dash is bad are the wire going to it ( The 1 you grounded ) is broken.

If the gauge is reading low oil pressure, Then unscreew the oil sending unit on the engine just behind the distributor, were you took the wire loose from. Then install a OIL PRESSURE GAUGE into it's place. Then start the engine and read the pressure, Now compare the reading on the dash gauge to what you are reading on the oil gauge.