The G-21 was powered by a five-cylinder M-11Ye radial engine, installed with its cylinders projecting out of the smooth cowling for cooling and driving a two blade propeller. Behind the engine the fuselage was smoothly ply covered. The front of the cabin was over the leading edge, with panelled glazing that extended aft to about three-quarters chord. The G-21's cabin roof line extended rearwards unbroken, tapering only slightly to the tail, where a forward set, mid-fuselage mounted tailplane was braced on each side by a single strut to the fin-fuselage junction. The fin and unbalanced rudder had a curved profile; the rudder was broad and extended down to the keel.
