He received the Navy Cross for distinguished service while Master of the American merchant steamer SS City of Flint, seized by a German cruiser on the high seas on 9 October 1939 but returned to him in a Norwegian fjord on 3 November.

She departed Pearl Harbor on 12 March and staged at Saipan for the forthcoming invasion of Okinawa, acting as a part of a decoy task force that made feints against the southeastern coast between 1 and 2 April while the landings were effected on the western beaches.

Gainard operated as radar picket and fighter director ship throughout the bloody Okinawa Campaign, detecting enemy air raids, providing early and continuous information to friendly forces, and initiating interception with a Combat Air Patrol unit that found her controlling an average of 10 planes from dawn to dusk with the assistance of a fighter director team on board.

Based out of Newport, Rhode Island, her operations over the next 20 years have included nine deployments as an anti-submarine warfare specialist with the "Steel Gray Diplomats" of the 6th Fleet; several cruises to northern Europe for the training of midshipmen; amphibious warfare exercises along the coasts of Virginia and North Carolina; plane guard duty for aircraft carriers off Mayport, Florida; and combined 2nd Fleet exercises and anti-submarine tactics along the Atlantic seaboard and in the Caribbean.

This was only one of many operations in which Gainard made important contributions to improve the overall combat readiness of forces earmarked for the Allied command in defence of an outbreak during the Cold War.

Other vital tasks included schoolship duties for the Fleet Sonar School at Key West, Florida; participation in Operation "Mercy" with carriers Shangri-La and Antietam in rendering assistance to thousands of flood-stricken victims of Hurricane Carla off the Texas coast during September–October 1961; gunnery schoolship duties for the Fleet at Norfolk; and service as a unit of the Cuban Contingency Task Groups during the Cuban Missile Crisis of November–December 1962.

In May 1963, Gainard served as support ship on recovery station during the successful launching of Faith 7, the ninth and final Project Mercury manned space flight, piloted by Astronaut L. Gordon Cooper.