Gazzo began his film acting career in the 50s with a small role in Elia Kazan's "On the Waterfront", but he turned his attention to writing and teaching until 1971 when he returned to the big screen in "The Gang That Couldn't Shoot Straight". But few in the movie audience had heard of him until he burst onto the screen as Pentangeli, the subordinate who tells Michael Corleone, "Your father never trusted Hyman Roth" in Francis Ford Coppola's "The Godfather, Part II". Gazzo received an Oscar nomination as Best Supporting Actor for his work, and while he would go on to be a kingpin in "King of the Gypsies" (1978), Mike in "Fear City" (1985) and play a memorable role in "The Last Action Hero" (1993), none of his subsequent feature film roles equaled his "Godfather II" impact.
Gazzo was also much in demand on TV, and performed in numerous episodes as well as longforms. Often his parts were of mobsters, such as Salina, the gangster held hostage in "Blood Ties" for Showtime in 1986. He also received much applause for his work in "Brink's: The Great Robbery" (CBS, 1976) and as Marullo in the adaptation of "John Steinbeck's 'The Winter of Our Discontent'" (CBS, 1983).