The G-21A eight-seat commercial amphibian flew for the first time in June 1937 and was Grumman's first aircraft produced for the private and commercial market. It proved outstandingly successful and 20 were delivered prior to 1 October 1938, when many more were on order. However most of the 300 or so aircraft built went into military service, the US Navy receiving initially JRF-1s and the US Coast Guard JRF-2s in 1939-40.

Further series included JRF-1A aircraft, fitted for target towing and photography; JRF-3 fitted with anti-icing equipment and autopilot for use in northern waters by the US Coast Guard; JRF-4, a development of the JRF-1; JRF-5 (OA-9) for the USAAF and fitted for photography; and the JRF-6B navigational trainer. JRF-5s also served with the RCAF and the JRF-5 and -6 with the RAF as the Goose. Production of the JRF ceased in September 1945, but the type continued to serve as a general/utility amphibian with the US Navy and Coast Guard.

After the war McKinnon Enterprises began a conversion programme, replacing the original 335.3kW Pratt & Whitney R-985-AN-6 engines with four 253.4kW Lycoming GSO-480s and incorporating other refinements to produce executive transports. This programme was superseded by the turboprop-powered G-21C, D and G Turbo-Goose conversions, initiated in 1966. The latter is the current version, powered by two 507kW Pratt & Whitney Aircraft of Canada PT6A turboprop engines.