Scott,
By the way GELCOAT does stick to EPOXY.
The epoxy must be carefully measured and mixed. Must be fully cured. Must be NO BLUSH.
My specs would say: 100% solids - 1 to 1 or 1 to 2 parts premium epoxy, so that there is no solvent out gassing to mess with the polyester.
Then it should be sanded with 60 grit.
Gelcoat will stick to epoxy.
But why would you want to use gelcoat over perfectly good epoxy?