He held several teaching posts at academic institutions in the southern United States during his career, and also helped to found the Symphony of the Air orchestra.
Gillis also produced the radio program Toscanini: The Man Behind the Legend, which ran for several years on NBC after the Italian conductor's death.
In 1973 he joined the faculty of the University of South Carolina where he founded, and was chairman of, the Institute for Media Arts and was instrumental in establishing the Instructional Services Center.
Despite his administrative responsibilities, Gillis was a prolific composer, writing ten orchestral symphonies, tone poems like Portrait of a Frontier Town, piano concertos, rhapsodies for harp and orchestra, and six string quartets.
He assimilated popular influences in a simple and straightforward style aimed at communicating with his audiences through an emphasis on clear, accessible, melodic writing.