Gruber was one of the most prolific writers of pulp fiction, writing more than 300 stories for over 40 pulp magazines, as well as over sixty novels, and over 200 screenplays and television scripts.
He grew up on the family farm, and after a stint in the Army, Gruber took on various jobs, working as a bellhop and a ticket-taker at a movie theatre. After some success writing for agricultural trade magazines, he moved to New York in 1934 to try to make it as a full-time writer. He took several freelance writing gigs, including trade journal editor and correspondence school teacher. He finally got his big break doing detective "quickies" for the pulps. He wrote everything from science fiction to romance, but is chiefly remembered for his westerns and his detective stories. He even wrote for Black Mask, the most prestigious of all pulp magazines.
For the crime and detective pulps, he wrote a long string of short stories featuring smooth-talking crime-solving encyclopedia salesman Oliver Quade, some of which were collected in a book called Brass Knuckle. Quade was a much loved character, and when Gruber started writing longer works, many of the elements of Quade found their way into his detectve novels - quirky characters; fast-paced fun;, a detective with, at best, quasi-official status, and a gift for the gab. Interestingly, most of them revolved one way or another around books.
Gruber also created several two-man teams of private eyes such as Johnny Fletcher and Sam Cragg; Simon Lash and Eddie Slocum; and Otis Beagle and Joe Peel. Fletcher and Cragg were traveling con artists, slick rascals flogging a body-building manual. Fletcher was the brains of the outfit, while Cragg was the strongarm sidekick.Simon Lash was a cranky Los Angeles-based PI who collected rare books about the early frontier which often served as the "Macguffins" of the books' plots.
Gruber also wrote for the film and TV industry in Hollywood. It was said that he wrote over 200 teleplays (mostly in the western genre), and created several TV series, most notably Tales of Wells Fargo, The Texan, and Shotgun Slade. In 1967, Gruber published The Pulp Jungle, a collection of reminisces of his years as a pulp writer.