Gravel itself drains quite well and doesn't retain water, unless it is some really porous stone like limestone. But if the soil underneath is rich in clay and stays moist, them moisture can travel both ways thru the gravel, contributing to a moist environment under the vehicle. It's not the gravel - it's the soil. But if a paved driveway was installed, there would be a barrier between the moist soil and the vehicle, so in that sense the gravel is part of the problem, simply because it is not solid pavement.
Like the others, I find it hard to believe that this is a significant contributor to the overall problem of corrosion.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL