He later had a second career as a horror film actor, including a lead part in Dan O'Bannon's The Return of the Living Dead (1985).

Gulager's first major film role was in Don Siegel's The Killers (1964) with Lee Marvin and Ronald Reagan in his only movie role as a villain, followed by a supporting part in the racing film Winning (1969) opposite Paul Newman and Joanne Woodward; in Peter Bogdanovich's drama The Last Picture Show (1971); and opposite John Wayne in McQ (1974).

[4][5] His Cherokee nickname was given to him by his father for the clu-clu birds (known in English as martins,[4] like his middle name) that were nesting at the Gulager home at the time of his birth.

[7] In the spring of 1959, he signed with MCA-TV, where he appeared as Tommy Pavlock in the episode "The Immigrant" of NBC's series The Lawless Years, a 1920s crime drama.

In the fall of 1959, he appeared in the episode "The Temple of the Swinging Doll" of NBC's short-lived espionage drama Five Fingers, starring David Hedison.

On October 11, 1959, Gulager appeared as a U.S. Navy sailor in the "Appointment at Eleven" episode of Alfred Hitchcock Presents and again as an escaped convict in "Pen Pal" on November 1, 1960.

Late in 1959, he was cast as Beau Chandler in the episode "Jessie Quinn" of the NBC Western series Riverboat, starring Darren McGavin and Burt Reynolds.

The episodes portray Billy as a sympathetic character without resorting to the "misunderstood young man" theme used in such films as The Outlaw (1943) and The Left Handed Gun (1958).

Gulager portrayed Deputy Sheriff Emmett Ryker from 1964 to 1968 on The Virginian, the 90-minute Western series in which he starred with James Drury, Doug McClure, Lee J. Cobb, Roberta Shore, Randy Boone, Gary Clarke, and Diane Roter.

In 1977, long after his role on The Virginian, he appeared in an episode of Rod Taylor's unsuccessful NBC Western series The Oregon Trail.

In 1981, he co-starred as Angela Channing's long-suffering nephew Chase Gioberti, opposite Oscar-winner Jane Wyman, Lorenzo Lamas, William R. Moses, and Jamie Rose, in the pilot episode of The Vintage Years, which was later retooled as the primetime soap opera Falcon Crest.