With a receiver/bumper pull, the entire tongue load is cantilevered aft of the rear axle, so it puts the entire load on the rear axle, AND removes some load from the front axle as the truck nose tips up (slightly or more, depending on the situation). Mounting the GN forward of the rear axle will distribute at least some of the load to the front axle. How much would generally depend on the inverse ratio of the distances to the rear axle and the front axle, from the GN mount, but that is a simplification.
If you have heard of the trick with a bathroom scale for measuring tongue loads by using a bar on the scale and on a block and placing the tongue at some fraction of the bar length, it is the same principle. The closer to the scale you place the load point, the more load the scale sees.