If it is the GRP part only, a light rub with T-cut or Brasso (same thing) then a good polish with Mer or some such high quality car/boat/caravan polish. A good quality wax based polish is another way to go but it is harder work to apply.
Don't be over vigorous or enthusiastic with the T-cut. The matt finish on the GRP is a problem caused by sunlight affecting the gelcoat on the GRP. The gelcoat layer is quite thin and you may go through it.
Care-avan polish I've no connection to this company other than being very impressed with their products Their polish will give you the results your after I used it on my boat to bring back the shine on the hull which had been dulled by the action of sand rubbing the shine off.
Not cheap but well worth the cost for the finish you get.