There are a number of products available and I would hope a chandlery
would be able to help you with enough advice on what to use and how. If not them, then a paint
supplier. Products I personaly recomend are International and Altex Devoe.
If they are light and cosmetic only, then a good sand followed with an epoxy
primer. The use of Epoxy primer will ensure a very good adherence to what I will presume is an Ester based gelcoat. Allow it to dry, but not too long or it goes so damn hard, sanding
is a nightmare. Allow the time the manufacturer recomends. A light sand is all that should be required and then apply the top coat using a two pot polyurathane. I recomend the two pack poly as you are in hard wearing high traffic area's. For best results, spraying is best, BUT!!! you need a REAL respirator if you spray that stuff. I may be best to prepare the surface and mask out and then get an expert in to do the spraying. In fact, if you have no experiance with spray painting, I high recomend you get a pro to do it.
If you apply by brush, you need to roll the coat on and then brush the surface out to get a smooth coating. But it never looks as good as spray. A good sprayer should be able to make the surface look like gelcoat again.