Ed, you don't need it. Gelcoat was mainly used on new castings such as boats, camper shells, shower surrounds etc.
Usually a gelcoat was sprayed as the outer layer in the mold, (and the paint color could be mixed in), then the layers of fiberglass and resin added to desired thickness, it comes out of the mold smooth and colored. I'm just guessing on the waxed gelcoat, but may have been for an easier/additional mold release?
If you are just bonding/ modifing or repairing fiberglass, not needed at all. Some guys will work the fiberglass repair then gelcoat to seal it. In that case i would gelcoat bare glass, then do your bondo work. Really not needed if you work glass propperly. Then prime and block, once again some will say epoxy prime. To each there own.
Lol, and don't get me started on "My Corvette needs a new gelcoat after I stripped it" Factory Corvettes NEVER had a gelcoat!