I have called Gillespie and spoken with one of their paint reps. He said yes you can paint Gillespie top coat over a urethane epoxy primer. The condition being that it gets scuffed with sandpaper before hand. Epoxy primer especially if it sits for a while can tend to be smooth and not have much for a top coat to grab on to. When I painted the last restoration I scuffed the epoxy primer with red scotch brite. The top coat was urethane metallic high gloss car paint. It went fine.
The Gillespie primer has zinc and properties that should handle corrosion just fine. I ordered a gallon of black primer, chassis black satin, and two gallons of 30277 sand camo from Mark at Rapco. I had a 45 minute conversation with Mark. Great guy there.