Due to the oil crisis of the early 70s, boats were starting to use lighter layups and the search was on for better ways to balance glass and resin use in boatbuilding, at about the same time as GB moved from wood to FG in their boats. Hand layup was still the norm, resins were almost exclusively polyester. It wasn't till blisters became a pervasive problem that other resins were tried, such as vinylesters and, in high end shops only, epoxies. I don't know exactly when you could get a production boat built with epoxies instead of polyesters, but I am sure it never happened in GBs.