Seeman Gaddis was born on 26 January 1899, in Jersey City, New Jersey, son of Mrs. Anna (Hellam) Gaddis (now Mrs. Gorham), and the late James Gaddis. Appointed as a Boatswain in the US Naval Reserve on 12 February 1941, he was commissioned a Lieutenant in the Naval Reserve on 16 April 1942. He was promoted to the rank of Lieutenant Commander on 3 October 1945, and then Commander 1 July 1951.
Upon receiving his appointment in 1941, he was assigned to the District Intelligence Office, Twelfth Naval District, San Francisco, California, where he was serving when the Japanese attacked Pearl Harbor, Territory of Hawaii, 7 December 1941. He remained there throughout the most part of World War II, and until January 1945 when he transferred to the District Intelligence Officer, Seventeenth Naval District, Dutch Harbor, Alaska.
Returning to the United States in November 1949, he reported for duty in the District Intelligence Office, Ninth Naval District, Chicago, Illinois. Commander Gaddis retired in March 1961. He died in Santa Clara, California, on 2 July 1979.
Commander Gaddis had the American Defense Service Medal; the American Campaign Medal; the Asiatic-Pacific Campaign Medal; and the World War II Victory Medal.