Surely all polishes are abrasive to some extent?
...and waxes the shine only products?
I've used very fine 3M machine polish on GRP with excellent results. It can be worked without too much pressure and takes as little surface as you want. The result for me was a clean but VERY smooth shiny surface that I then waxed for protection. My ten year old GRP that hadn't seen much cleaning looked like new. It's a messy job and the polishing compounds are very hard to get off hypalon tubes (I guess it's clingy and the grit sticks to the irregularities). I wrapped my tubes in industrial packaging film before doing the whole boat.