The GL-21, like its predecessors, was powered by the 180 hp Hispano-Suiza 8Ab eight-cylinder water-cooled in-line V-engine, but the broad chord ailerons with their horn-balance areas lying within the wing planform were replaced by longer-span and narrower constant-chord ailerons. The rudder was enlarged and the series model introduced additional undercarriage bracing struts. Armament comprised two 0.303 in (7.7 mm) Vickers machineguns in the fuselage.
One GL-21 was purchased by Finland in 1923, followed by a further eighteen in 1924, an additional aircraft later being assembled in Finland from spares, and these remained in Finnish service until 1931. A total of 30 single-seat fighters of this type was built.
Span: 30 ft 10 in (9.40 m)
Length: 21 ft 1.125 in (6.43 m)
Wing area: 202.37 sq.ft (18.80 sq.m)
Empty weight: 1,455 lb (660 kg)
Loaded weight: 2,116 lb (960 kg)
Max speed: 149 mph (240 kmh) at 3,280 ft (1,000 m)
Range: 280 mls (450 km)