For his screen performances, Gunn is best known for his roles as Clotho in WUSA (1970), Bumpy Jonas in Shaft (1971) and Joe Kagan on Little House on the Prairie (1977–1981).

After serving in the United States Army, he attended graduate school at the University of Kansas, earning a master's degree.

He performed many Shakespearean roles in Joseph Papp's Shakespeare in the Park, winning an Obie Award for his portrayal of Aaron in Titus Andronicus.

Gunn appeared as atheist shop owner Carl Dixon in six episodes of Good Times, as boxer-turned-farmer Joe Kagan on Little House on the Prairie, and as "Moses Gage" in Father Murphy.

His final acting role was as murder suspect Risley Tucker in "Three Men and Adena", an episode of Homicide: Life on the Street.

In 1970, they had a son, Justin, who became a musician and composer in the Copenhagen-based band, The Reverend Shine Snake Oil Co. Gunn died from complications of asthma in Guilford, Connecticut on December 16, 1993, aged 64.