If you want to apply layers of woven glass or chopped strand mat for additional strength on the OUTSIDE, then grind off all of the gel coat in that area first.
Personally I wouldn't do it that way unless there is severe body damage, e.g., large holes and large cracks. I would add the glass layers to the inside and simply repair any gelcoat damage on the outside surface.
If you are repairing the front lower body, at least on my Trillium
there was stone damage resulting in chipped gelcoat and some very small holes through the fiberglass. I fixed the damage by first adding chopped strand mat to the inside surface, then repairing the gelcoat. I also painted the lower front section with chip guard paint