What does "GG" mean on the federal pay schedule?

On the U.S. federal pay schedule, GG is a pay plan used exclusively for employees of the U.S. Mission to the United Nations (USUN) and the Foreign Service Institute (FSI).

Employees on the GG pay plan are excepted service employees, which means their jobs were not subject to open competition. The GG pay plan is generally identical to the General Schedule (GS), which is the applicable pay plan for the majority of government employees. The difference in classification is made for bureaucratic reasons, in order to distinguish employees of USUN and FSI from other government employees. Only minor technical differences exist between the GG and GS pay plans.