How is your oil level?
If that’s good, then it’s either a bad gauge (instrument cluster), bad sending unit, wiring issue between the sending unit and cluster, or a mechanical issue.
The gauge sending unit is in kind of a crappy location by the distributor. Ideally, you’d swap the sending unit out for a mechanical gauge to see what the real pressure is.
Or swap in a junkyard cluster and see if the issue is corrected; you’d swap over the odometers if you were keeping it.
Or swap out the sending unit. There is an actual diagnostic that GM uses to check the sending unit function, but I’m not sure if you’d need a special ($$) Kent Moore tool or not.