If you have to refinish it, do as Dave has outlined. Once you rough up the gel coat and prime it it's just like painting any other item.

However, you may want to try buffing it up after a good, thorough cleaning. A mild compound and a variable speed buffer should do it unless it's really deteriorated. If you've got any boat maintenance outfits in your area you can check with them, polishing up gel coat on boats is common practice.