Why might I have low oil pressure?

If its idling very low you will have lower oil pressure. Get the truck why its idling low . If you rase the idle up is your pressure normal?

Your idle is computer controlled so don't mess with that now. You need to first use a mechanical gauge to read the actual pressure and determine if it really is to low or just a faulty gauge. You also want to look at the oil and determine if it's been diluted with gas or not.

no gas in oil, no mechanical pressure gauge on site. when in motion the truck does have normal oil pressure, but, when you come to a stop, it will stall out. also when starting, need to give it gas or it will stall.

What does the stalling have to do with oil pressure? If the idle is dropping that low, it would be normal to see an oil pressure drop also.
Let's address the stalling.
First eliminate all vacuum or air leaks in the system. You may have a bad IAC ( idle air control) but try cleaning the throttle body first. Remove the intake snorkel, have someone hold the throttle wide open for you and scrub the back side of the throttle plate and surrounding bore with an old tooth brush and some carb cleaner. Be sure to spray some into the small holes next to the throttle plate. That should help stabilize the idle. If it still has a problem, replace the IAC.

This truck needs the oil psi good to keep the fuel pump running. Check oil psi and check the idle.

No, it does not require oil pressure to keep the pump running.

Wrenchtech is correct this year does not need oil psi to keep the fuel pump running I just checked the wiring older gm trucks need good oil psi. Carey on with checking idle and oil psi @ the oil switch

The older trucks do not require it either. The oil pressure switch is a redundant power supply for the fuel pump relay on the older trucks. It will still run with either the switch unplugged or the fuel pump relay removed.