He enlisted in the United States Army and worked as an intelligence officer during World War II in the Pacific Theater.
[1] After completing his military service, Gillis moved to Los Angeles and took a job writing for radio shows, including the dramas The Whistler and Let George Do It.
He moved into television scriptwriting and earned his first credit — for an episode of Racket Squad, a series that starred Reed Hadley — in 1952.
His scriptwriting was prolific and varied, and over the years, he worked on shows such as Lost in Space, Hawaii Five-O, and Knight Rider.
Gillis was married to the former Patricia Cassidy, a fellow actor whom he met during his brief acting career at the Barter Theatre, until her death in 2003.