How do you convert reid vapor pressure to gage vapor pressure?

Under the ASTM Method D 323 (Reid vapour pressure), it is the absolute vapour pressure exerted by a liquid at 100°F. The higher this value, the more volatile the sample and the more readily it will...

They are the same when the air is saturated and the relative humidity is 100 percent.

Vapor pressure is the pressure produced when vapor particels above a liquid in a sealed container, collide with the container walls.

its just under 0.5 pounds.

No. It depends on the temperature. The only time you will really consider Pext is when you want to determine whether the substance will boil, which is when Pext = Pvap.