Can an oil pan gasket leak cause low oil pressure, even when the oil level is not low?

No.

But, it can cause a mess on the floor/ground...

Are you experiencing low oil pressure?

Causes of low oil pressure and repairs:

Low oil level - bring to proper level
Engine running hotter than normal will cause significant oil pressure drop - check cooling system
Bad dashboard gauge - check oil pressure with mechanical gauge
Bad oil sending unit - replace sending unit
Relief spring stuck open or weakened - replace spring and/or piston
Worn bearings - replace bearings (major engine overhaul)
Oil diluted with gasoline from leaking injector - check oil and replace injector
Plugged strainer - drop pan and clean or replace strainer
Worn oil pump (this is least likely cause of low pressure) - replace pump

the onlt time the pressure seems to drop is after i am been driving 70-75 mph on the expressway for more like an hour or more. then after the car sits for a long time off, it goes back to normal.

Sounds like your oil is just thinning out from the heat of heavy running. This is most often noticeable when coming to a stop at the bottom of a freeway exit ramp. The problem in that case is heat soak. The engine cooling system stabilizes on the highway with lots of air rushing through the radiator. Then you come to a stop. The water pump is turning much more slowly, there is no air flowing through the rad, and the cooling system cannot deal with the residual engine heat, so the oil thins and you observe the pressure drop.

Also, what viscosity oil are you using? With the mileage on your engine, you might benefit from running a heavier weight oil. I run 15w40 in my '95 year round with no winter starting problems.

Before we move onto a different viscosity.

When was the last oil change?
Do you happen to know the oil and filter that was used?.. if so please post the names.