Easiest and cheapest is to scrub the non skid with a degreaser type cleaner (simple green, Zep, etc).... use a stainless steel brush to lift the crud and algae out of the texture. Try a fairly small area first, If that's not working to your satisfaction, go to the more aggressive solutions suggested by the others.
I'd not use a metal brush: Unless you're a plastic surgeon (who doesn't care about his or her gelcoat). I'd not use a metal brush anywhere near fiberglass that you care about. If a soft, clean brush won't do it, then use warm water, then soap, then get more aggressive with cleansers.
You can use Crazy Kleen or some other similar cleaners; and, maybe even Acetone -- very quickly with clean cloths (and little concern for your future as a possible cancer patient) :^))).
ANOTHER culprit which can make a non-slippery fiberglass surface very difficult is if you put vinyl treatment or Armour all or spray waterproofing for canvas.