[mikeyk]

Over the years of restoring glass deck surfaces on my work boats, I have worked with a number of different materials to achieve a good non-skid gelcoat. Silica sand sprinkled on first tacky coat coat of unwaxed gelcoat and then recoated with waxed gelcoat. Also, ground walnut shells instead of sand.
Drawbacks with sand is the work and materials involved when you have to grind it off. Walnut shells are very sharp and aggressive,making it a good surface, but it can cause skin abrasions and wear and tear on gear.
Cabosil thickened gel coat applied with a fluffy roller is also a good alternative, however, the "peaks" of hardened gelcoat are like embedded broken glass (another good source of abrasions and gear wear and tear) and must be lightly sanded before use.
My choice is coarse Kosher style salt. Sprinkle it evenly over uncured gelcoat, and wash it off after the gelcoat cures. When you have to sand it off, you are only grinding hardened gelcoat.