C. Class D.
From: FAA-H-8083-25B "Pilot's Handbook of Aeronautical Knowledge"
47. (Refer to FAA-CT-8080-2G, Figure 75.) The airspace surrounding the Gila Bend AF AUX Airport (GBN) (area 6) is classified as
Page 15-2, Class D Airspace
"Class D airspace is generally airspace from the surface to 2,500 feet above the airport elevation (charted in MSL) surrounding those airports that have an operational control tower. Unless otherwise authorized, each aircraft must establish two-way radio communications with the ATC facility providing air traffic services prior to entering the airspace and thereafter maintain those communications while in the airspace."