I used the "sugar-deck" method old surfboarders used. I applied it over the existing gelcoat layer after properly preparing the surface (MEK, light sanding, Acetone)

Masked off (with tape) the area to be applied.

Sprayed on gelcoat like you normally would

Blow (or spinkle) sugar crystals on the gelcoat before it sets up, and remove tape.

When gelcoat is hard, just wash the section with water. The sugar will dissolve away, leaving a rough gelcoat surface.