Tony, GRP is a more generic acronym for Glass Reinforced Plastic. It kind of throws a blanket over polyester and epoxy composites.
Mitch, the rationale for putting GRP over wood is that many people are comfortable working with wood to get the shape and basic structure that they want, but stressed (bent) plywood tends to split apart and let water get into the grain if not reinforced. Paint doesn't get the job done by itself. So that pushes people to do a thin glass cloth over wood. Same reason to glass a woody; keeps water out of the wood grain by reinforcing the sealer coat. W/o the glass the wood moves, the sealer coat cracks and fails and the water gets in. Kind of hard to get a woody look without any wood. Sure, some faux wood paint finishes can be pretty convincing at 10 ft, but I have never seen any look as good as juneaudave's work in the real thing.
If the wood is eliminated additional skills are needed to shape and fair the foam, it is a messy job; and in order to get adequate structural and puncture resistance additional plies/weight of cloth are needed. This turns what many consider to be a wood working project into a fiber glassing project, and many people just aren't going to go down that path. But then again you probably already realized this when you asked the question.
Lastly, many people have been successful using prefabricated FRP (fiber reinforced plastic... an even less specific tag) sheets or Filon, some even w/o a plywood substrate.