GB is the abbreviation of the National Standard of the People's Republic of China, as a member organization representing the People's Republic of China in the International Organization for Standardization (ISO) and the International Electrotechnical Association (ffiC). GB standards are issued by the National Standardization Administration. The mandatory standards are prefixed with "GB", and the recommended standards are prefixed with "GB/T".
To see more details about GB6170, GB6171, GB6175, and GB51, please kindly check equivalent norms for Hex Nut ISO 4032, Hexagon Nut ISO 4034, and Hex Nut ISO 4036.