On October 12, 1929, he was commissioned a second lieutenant in the Air Corps Reserve after graduating from Advanced Flying School at Kelly Field, Texas.

Shortly after the attack on Pearl Harbor, General Gerhart joined the newly formed Eighth Air Force.

In early 1946, Gerhart returned to London and Paris for a year as air adviser to the American Delegation participating along with the major allies in drafting the Balkan and Italian peace treaties.

On return to Washington in January 1947, he served successively as director of the Legislative and Liaison Division, chief of statistical services in the Office of the Comptroller, and as chief of staff of Joint Task Force Three which conducted the first thermonuclear tests at Eniwetok Atoll in the spring of 1951.

Gerhart's decorations included the Silver Star with two oak leaf clusters, Legion of Merit, Distinguished Flying Cross, Bronze Star, Air Medal with two oak leaf clusters, French Croix de Guerre with Palm, and Belgian Croix de guerre with Palm.