Gagner was born in Cleveland, Ohio and grew up near Daytona Beach, Florida, where he was a multi-sport all-state athlete at Seabreeze High School.

[4] During his college career, Gagner primarily played offensive guard, but also appeared at center, defensive tackle, and linebacker due to his combination of strength and speed.

[10] He reported to the Steelers' training camp in July, but after one practice, the team doctor determined that Gagner had not recovered from his injuries and recommended that he postpone his return.

[8] Though the team put him on a physical therapy program, his hip continued to give him problems, and Gagner missed the entire 1970 season.

[10] Gagner came out of retirement in 1974, playing in a backup role for the Jacksonville franchise of the World Football League (WFL) for two truncated seasons.

[10][12] The WFL folded in 1975, and Gagner signed a free agent contract with the NFL's expansion Tampa Bay Buccaneers in March 1976.

[14] In one last attempt to continue his career, Gagner tried out for the Toronto Argonauts of the Canadian Football League, where the team doctor warned him that returning to the field would accelerate the degeneration of his injured hip.

After football, Gagner worked several different jobs, including as a taxi driver, security guard, and substitute teacher, before deciding to make use of his college degree in commercial art.