TotalBoat Gelcoat's finish can be reduced from gloss simply by wet sanding the surface. The grit of wet sandpaper will define the finished gloss level. The finer the grit (higher grit number), the higher the gloss. A good starting range of wet sandpaper is 1500-grit to 4000-grit.
It's important to understand that reducing the gloss of a gelcoat material reduces the UV reflectiveness, causing it to absorb more UV light, leading to faster oxidation. Concurrently, it is also more prone to staining in the microscopic grooves and cavities from the wet sanding.