Hi Paul,
At first GRP does give of fumes (styrene).I don’t think grp out gasses as such as it is a chemical reaction that cures the resin. GRP can take upto two weeks to cure fully and attain full strength but that shouldn’t be a problem to you what may be a problem is the mould release that they have used, this maybe a wax or a synthetic mould release that is used in the industry. The product keeps some of this on it when it comes out of the mould so you should clean it well if you are fitting the vinyl straight to a gel coat finish.
Steve