What causes zero oil pressure reading in a 2003 GMC Sierra FWD?

The gauge may have gone bad, the oil pressure sending unit may have gone bad, the oil pump may have quit. I suggest you find the oil pressure line coming off engine, put another oil pressure gauge on it and then just crank the engien to see if you get any oil pressure whatsoever. If you don't, then I would check the oil pressure sending unit next before I tore the oil pan off to examine the oil pump itself. Whatever you do, do not run the engine with zero oil pressure otherwise you will seize up the engine and that will cost big bucks to repair.