"Gold that is not solid is normally marked GF (Gold Filled), HGE (Heavy Gold Electroplate), HGEP (Heavy Gold Electro Plate), E (Eletroplate), GE (Gold Electroplate). It isn’t marked FG as for it’s purity or as for it being plated.
The ‘FG’ stands for the makers Initials, or the Manufacture’s logo. It's the trademark of Frederick Goldman, Inc."