All you should do is remove the gloss with a scotchbrite pad. To use sandpaper can be more risk/trouble than it is worth. Gloss is the enemy, but the gelcoat should be left as intact as it can be.
As far as paints are concerned, you can use any paint that suits other than possibly laquer. Laquer's volatile solvent is strong enough to damage some composite coats, not to mention the results are not worth it.
Acrylic urethane is my personal favorite. It does not damage undercoatings and is the most durable and glossy. This and other comparable finishes can be found at automotive supply shops such as NAPA. Observe all precautions and procedures and you will have a truly hot lookin ride