Working with gel coat is a lot like working with fiberglass resin. In fact that's pretty much what it is, is just tinted resin. If you have never worked with fiberglass resin before, think about it as a thick paint that you have to add an activator/hardener to in order to get it to cure.
Now with gel coat, if you are trying to fill a crack, what you need to do is either thicken up the gel coat with a product called cabosil if you are working with raw gel coat. Or, just get a patch kit, something like this in this link below;http://www.westmarine.co...er=1934454&langId=-1
Which is pretty much pre-thickened gel coat.
When filling cracks, what you do is grind out the crack a bit with a small dremel tool. Clean it with acetone. And you build up the gel coat in layers, and once it has cured, sand it flush, buff it, and wax it. There are also 2 types of gel coat, gel coat with wax, and wax free. The reason why is standard gel coat will not fully dry in air, it has to be covered. It will get firm, but it will not fully cure, and will always be sticky. To get around that they make gel coat with wax in it, and the wax rises to the surfaces as it cures, to block out the air.
If you are going to do this yourself it is important to know if your working with wax free gel coat, or gel coat with wax in it. In most cases to fill a crack it takes 2 or 3 layers of gel coat. If you are working with waxed gel coat you would have to sand in between coats in order to knock off the wax. If you are using wax free gel coat, you can layer 1 coat on top of the next. But after the last coat, you must cover it either with saran wrap. Or, what they call PVA, which is a poly vinyl coating that drys like saran wrap.
It's pretty involved when working with the raw materials. The patch kits that are sold at west marine take a lot of the techie stuff out of it. There is definitely a learning curve to working with the material.