The G.12 was an all-metal low-wing cantilever personnel transport aircraft. It had three radial engines, one mounted on the nose and the other two in wing-mounted nacelles. The engines drove three-blade feathering metal propellers. The mainwheels of its landing gear retracted into the nacelles; the tailwheel was fixed. The flight deck and cabin were fully enclosed. Access was via a portside access door aft of the wing.