GRP is an acronym for 'glass reinforced plastic' as Gary stated above. I've most often seen it used in British publications, I expect it has something to do with "Fibreglas" being a trademark. In North America, the material usually seems to be referred to as fibreglass
Regarding problems to look for, search through the various threads on Sailnet for topics like osmosis, delamination, coring, etc. There is a lot of good information here.