A gelcoat should not be applied over an old gelcoat as it will peel off over time. Even if you sand the old gelcoat. To apply a gelcoat/topcota, you would need to remove the old gelcoat and then apply a new resin and fiber laminate. Another option  would be to sand the old gelcoat, then apply an epoxy primer and finally 2-3 coats of 100 g/m2 each of a high-resistance polyurethane paint.