It’s similar to painting just about anything in that preparation is key. Clean off the stock with a solvent such as lacquer thinner or use wax and grease remover. Sand the gelcoat to create some tooth for the paint to adhere to using nothing finer than 320 grit. 240 or even 120 is fine. Then clean again and mask before applying your paint of choice according to the manufacturer’s instructions. People have used all manner of rattle can paint with various degrees of success/durability. We use a high end catalyzed epoxy paint and it has excellent wear resistance. Some have used bed-liner type material to good effect. Just remember… don’t bake it.