His films included Has Anybody Seen My Gal? (1952), Francis Covers the Big Town (1953), The Girl Rush (1955), Invasion of the Body Snatchers (1956), The Strange One (1957), The Brothers Rico (1957), Some Came Running (1958), Cat on a Hot Tin Roof (1958), The Remarkable Mr. Pennypacker (1959), One Foot in Hell (1960), Underworld U.S.A. (1961), The Young Savages (1961), Ada (1961), Toys in the Attic (1963), Cattle King (1963), The Sand Pebbles (1966), In the Heat of the Night (1967), Hour of the Gun (1967), Death of a Gunfighter (1969), Airport (1970), Lucky Luciano (1973), and Funny Lady (1975).
In 1964, Gates was nominated for a Tony Award in the category Actor, Supporting or Featured (Dramatic), for his work in A Case of Libel.
On television, Gates had numerous roles on such anthology drama series as Philco Television Playhouse, Alfred Hitchcock Presents, The Twilight Zone, Goodyear Television Playhouse, Kraft Television Theatre, Studio One, and Playhouse 90. He continued to make dozens of guest appearances in a wide variety of primetime series, including Bonanza, Route 66, The Defenders, Rawhide, and Twelve O'Clock High. He played the role of Secretary of State Dean Rusk in the 1974 teleplay The Missiles of October, and played President Herbert Hoover in the 1979 miniseries Backstairs at the White House.
Gates had a long career in film, television, and theater. He appeared in the Broadway productions of First Monday in October (1978), The Highest Tree (1959), The Carefree Tree (1955), The Taming of the Shrew (1950), The Teahouse of the August Moon (1953) Bell, Book and Candle (1956) and A Case of Libel (1964). Gates played Polonius opposite Sam Waterston in a New York revival of Hamlet. He starred in the 1976 Broadway play Poor Murderer, which is about an actor who questions whether or not he, who is playing Hamlet, actually killed the actor playing Polonius, or if it was just a dream.
His notable roles include H.B. Lewis on daytime's Guiding Light and Doc Baugh in the film version of Cat on a Hot Tin Roof. He played the role of Lewis from 1983 to 1996 and won the Emmy for Outstanding Supporting Actor at the 1985 awards. (He had previously played the role of District Attorney Eric Van Gelder on Guiding Light in 1977 and 1978.) Gates is also remembered for his role in the film version of In the Heat of the Night, where his character is part of a crucial scene involving a slap.
Gates died of leukemia in 1996. He was survived by his wife and a sister.