Gary's correct - GRP is another term for regular fiberglass.
The Vindos are reputedly quality vessels - but I'm thinking they may be glass hulls with wooden deck and coachroof??? If that's the case then the deck requires very careful inspection. At that vintage things may have begun to leak and/or rot.
I believe Ferenc Mate's book "The Worlds Best Sailboats" featured the Vindos as one of his picks.