Well, I have to agree with Martin on this one. We have a big named boat builder here in town and he uses nothing but polyester and mostly a chopper bun instead of cloth! And he builds speed boats the same way as fishing boats. Never have seen him use epoxy. I have never used it and in all my reading it is reserved for things like stress-skin race car bodies, aircraft parts and those funny car bodies that go 350mph and weigh 3oz! No doubt about it, epoxy reinforced carbon fiber is bullet proof.
Now about gel coat. Polyester resin used in laminating the cloth hardens very hard. It is a bear to finish for that reason. On the other hand, gel coat is formulated to be thixotropic - means thick coats stick on vertical surfaces - and it cures to the consistency of sanding primer. A gel coated surface can be treated exactly as you do a layer of PPG K36. If you want a perfect finish on a 'glass part, it must be gel coated. It has the added benefit of carrying permanent color so as Martin says, boat builders spray their color scheme in gel coat into a boat mold, lay in the fiberglass and pop out a perfectly finished boat shell needing no further finishing.
There are various grades of gel coat for various purposes. One is a tooling gel which is usually bright orange and is used to coat the inside surface of a fiberglass mold so it can produce many parts without damage to the valuable surface. Here are a few snap shot of a part I made recently showing;
a prototype made of wood plaster, whatever
The part finshed, painted and polished to perfection so I can get a perfect mold from it
The fiberglass mold finished inside with tooling gel and with parting lines so it can be taken off the part!
And finally the finished part with a thick white gel coat ready to finish sand and paint
Had to make the part in two pieces and epoxy it together.
I can tell from your questions you haven't read the educational sections of the web sites I linked above. Please do, they will answer a lot of your questions a lot better than I did. Fiberglast Co. sells how-to videos that are very informative. They also sell everything you need including the proper mold release agents, gel coats, resins and fabrics to do the job right. I buy all my stuff form them. I am a power ebaY user but I wouldn't buy my fiberglass materials there. WAY too much work involved to use inferior product and believe me there is a difference in quality in fiberglass materials.