Gelcoat / Spheres / Sand / Salt / Treadmaster
(1) There are people that do actual restoration of gelcoat non-skid but I can;t tell you who to call or give you any idea of price.
(2) Non-skid in paint is a good solution.
(a) plastic spheres in paint: work well, don't hold/trap dirt, Gentle on clothes/skin
(b) sand in paint: best traction, holds dirt, can draw blood/rip clothes
(c) course salt sprinkeled on wet paint (hard epoxy) - dissolves out with water rinse: great traction, can be very/too sharp
Note the spheres are sold by interlux, it is not real agressive but it works well and is my favorite. We have old treadmaster that is painted over with one part modern marine paint with the spheres mixed in before brushing. have been using the spheres for about 10 years and like it/them.