Geographically Separate Unit (GSU) is a United States military term describing a base that is physically separated from, yet not autonomous of its "parent" base. GSUs are "owned" by their parent organization and are typically quite small.
Assignment to a GSU can be a mixed blessing. The distance from the parent organization means a more relaxed atmosphere. However, it also means that members assigned to the GSU have to travel to the "parent" base for most military services such as the Commissary, Base exchange, or Personnel Unit.
RAF Feltwell, subordinate to the 48th FW of RAF Lakenheath, is an example of a GSU.
A GSU should not be confused with a detachment. Detachments are sub-units assigned to a different base from the parent unit. Detachments from one military branch can be assigned to a base belonging to a different branch. For instance, the 374th Communications Squadron from Yokota AB has a detachment at Camp Zama.