(polyester) gel coat is thickened resin* and is brushed into a mold and allowed to cure before the fiberglass. This gives the piece a good surface and prevents any fibers from showing through and being seen/felt at the surface. Gel coat is purchase as a separate item from the resin and is catalyzed with MEKP the same. Here is a good place to look for materials: http://www.shopmaninc.com/polyesters.html
Here is a good information source: http://www.fibreglast.com/contentpages-Learning-Center-286.html
In most case, you do not need gelcoat. If you are planning to paint the cast, no gel coat needed. It would be hidden behind several layers of primer and paint. You will not need gel coat for the case mold described above. Gel coat is need on things that you don't want to paint like a boat hull. It comes in many colors, and if you have a good mold, you can go without paint.
Many people will tell you to mix Bondo or plaster into resin to make gelcoat. It is cheap, and it will work, but not as good as just buying a small can of gel coat. The idea is to give you a sandable surface if you don't have a good mold.
* All 2 part polymers (Polyester, epoxy, urethane, silicone) have a resin side, and a catalyst side. So be careful how you through around the word "Resin." These days "Resin" generally means urethane casting resin.
