[4] During high school and college, he worked various jobs, including radio hosting at WFOY-Radio, contributing as a sportswriter to the St. Augustine Record, and announcing shows at Marineland.
It was during his directorship that St. Augustine's 400th anniversary was celebrated and the Great Cross commemorating the founding of the mission was installed.
[6] Gannon became a faculty member in the history and religion departments at the University of Florida in 1967, beginning a teaching career of nearly 40 years.
His first two books, Rebel Bishop (1964) and The Cross in the Sand (1965) explore Florida's early years of settlement.
[9] His 1990 book Operation Drumbeat discusses the history of Germany's early U-boats and became a national bestseller.
[11] He was a member of the National Park Service Advisory Board for Southeastern States from 1972 to 1977 and served as its chairman from 1976 to 1977.