His work can be seen on such popular programs as Bronco, Wanted: Dead or Alive, The Westerner, The Rifleman, Checkmate, Cain's Hundred, East Side/West Side, Route 66, Stoney Burke, Combat!, The Man from U.N.C.L.E., Honey West, I Spy, Mission: Impossible, and Batman among many others.
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.
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.
In the early 1970s Gries directed a variety of films, from the 1970 Jason Robards and Katharine Ross May–December romance drama Fools to the 1971 science-fiction telefilm Earth II starring Gary Lockwood and Anthony Franciosa.
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.