Gel coat is used to block UV rays from destroying your fiberglass. Gel coat is polyester resin with pigment in it. The pigment provides the protection. When a boat is laid up in a mold the mold is first waxed, then the gelcoat goes down, then the glass. Now understand that the gelcoat used in this process contains no wax. And of course the wax is what is used to create an air proof film on top of the resin to allow it to cure. Resin will not cure if it is exposed to air. Laminating resin contains no wax. That way the portion exposed to air never cures so one can come back the next day and lay fresh glass on it and still have a "primary" bond.
This "primary" bonding business is important. Polyester resin produces a perfectly good "primary" bond. In other words; when all the resin is layed up wet at the same time and cures at the same time then the bond produced is primary and strong. Polyester resin has very poor "secondary" bonding characteristics. In other words; if you stick fresh poly on something that is a solid surface aka...wood, fiberglass, (including glass using polyester resin,,,this makes no difference) then you will have a secondary bond, and a poor bond.
So, polyester resin and gelcoat are well suited for making hulls in boat molds. When this is done "ALL" the resin cures at the same time giving you a hull that is essentially one solid substance. Primary bond. The gelcoat will cure as soon as glass is layed on top of it, there-by cutting it off from the air.
You can paint gelcoat onto something and it will protect the glass underneath it. If you prep the surface well it will stick ok. However, gel-coat is a pain in the ass to work with. If you spray it then you only have a small amount of time before your gun becomes full of solid resin. You'll have to completely disassemble it and go through it very thoroughly. If you roll it there is no way to get a nice smooth even surface. I've tried. But it will be good enough for a work/duck boat. The fumes are horrible. Matching colors is very difficult. Quite an artistic en devour. Don't expect to ever mix up a batch of the same color twice.
Paint is a much better choice for top coating a fiberglass boat. It comes in a variety of colors. Is easy to work with. Depending on what you use it can provide wonderful protection. And it really is the way to go. If you're using gel-coat because you think your providing some super duper turbo awesome protection your a bit mistaken. The only really great thing to say about it is that you can pile it on real thick. But. To do this you will need to apply several coats. This will require you to sand the entire boat between coats to remove the wax from the surface of the cured layer. Not pleasant.
If you want some really amazing protection try barrier coat. 2000-E. Comes in grey and white. You follow the instructions and alternate between colors to ensure good coverage. This stuff is epoxy based. (which has amazing secondary bonding characteristics). If you paint your boat with this stuff it will be impervious to everything. Bullet proof. Then top coat with some paint.