Gries both wrote the screenplay and directed the 1959 Jack Buetel western Mustang! before concentrating his efforts exclusively on television for almost a decade. In a triumphant return to cinema, Gries wrote and directed what is generally acknowledged to be his masterpiece in either medium, the 1968 western Will Penny, which starred Charlton Heston in the title role. It was based on an episode of the TV series The Westerner that Gries wrote and directed in 1960, entitled "Line Camp".
In 1966, Gries created the popular action-adventure series The Rat Patrol. Gries wrote and directed the pilot episode, "The Chase of Fire Raid." The 1966–68 series boasts 56 thirty-minute, color episodes produced over the span of its two-season run on ABC. The series focused on the oft-overlooked North African Campaign and episodes invariably pit the ragtag Rat Patrol — a four-man Allied force led by Christopher George as Sgt. Sam Troy — against the German Afrika Korps led by Captain Hans Dietrich, played by Eric Braeden (then still using his original name Hans Gudegast).
Gries subsequently made two other films with Heston: the 1969 gridiron drama Number One and the 1970 drama The Hawaiians, which was based on James Michener's sprawling 1959 novel, Hawaii (not to be confused with the 1966 film based on a section of the same book). In 1969, Gries co-wrote and directed Jim Brown, Burt Reynolds, and Raquel Welch in the controversial western 100 Rifles.
Gries' 1970s work failed, however, to earn the critical acclaim that welcomed Will Penny. The most successful of his later projects was Helter Skelter, a 1976 TV movie based on Vincent Bugliosi's 1974 true-crime book detailing the crimes and trials of the notorious Charles Manson Family.
During post-production on his final film The Greatest (1977), a biography of boxer Muhammad Ali (in which Ali also played himself), Gries collapsed and died of a heart attack while playing tennis. He was 54 years old.
He is the father of actor and director Jon Gries (who appeared under the name Jon Francis in the film Will Penny as a child actor) and the brother of Buddy Charles a/k/a Charles Joseph Gries, who was a pop and jazz vocalist and pianist in Chicago.[3][full citation needed]