I've also been interested to know how the gates were painted. I believe that the primary color was actually painted first, then the lettering. I'm only going by what I've observed on patina'd trucks here, and every single one that has fading of the lettering has the primary color showing through.
Another thing that may allude to how the feathered edge was "achieved" is that only the inset was painted, not the radiused edges. I could image that a mask laying on top of the gate would then have depth between it and the inset so that when the lettering color was sprayed, it would result in the feathered edge.
Picture of my untouched original '68 gate for reference.
