A composer, critic, educator, and pianist, Gassmann is best-known for his ballet scores and the edgy electronic sounds he created with collaborator Oskar Sala for director Alfred Hitchcock's classic film The Birds. Gassmann received a degree in 1930 from St. Mary's College and continued his studies at the University of Rochester, New York, receiving his master's in music degree from the Eastman School of Music in 1931. Gassmann went to Berlin to study with Paul Hindemith at the Musik Hochschule from 1931 until 1936. One of his classmates was Oskar Sala, an early performer on the early electronic keyboard synthesizer called the Trautonium. Gassmann also studied privately with Remy den Hearynck, Eugene Goossens, Roger Sessions, and Isidor Philip.
Before America entered the war, Gassmann moved to Chicago, IL. He was appointed professor of theory and composition at the Orchestral Hall Preparatory School of the Chicago Symphony Orchestra from 1940 to 1946. Simultaneously, he was the music editor and critic for the Chicago Times from 1941 to 1947, wrote for the journal Modern Music, was director of the Composers Concerts and Seminars and a lecturer at the University of Chicago from 1942 to 1945, and director of the School of Music at Elmhurst College from 1943 to 1945. In 1946, Gassmann was commissioned by choreographer Ruth Page to create a score for the Ballet Russe de Monte Carlo. This is the well-known ballet Billy Sunday, which premiered in N.Y.C. on March 2, 1948, and was nationally televised decades later in 1983.
Gassmann then moved to Strasbourg, France, with his wife, Marthe Loyson, whom he married in 1937. He collaborated with Sala on several projects, including the ballet Paean, choreographed by Tatjana Gsovsky, which premiered in Berlin on May 29, 1960. Gassmann returned to live in Costa Mesa, CA, and created the ballet Electronics (1961), which featured ballerina Violette Verdy, for choreographer George Balanchine. Like most of Gassmann's works (the chamber music, Symphonic Suite, Ave Argentoratum for concert band and chorus, the ballets, and various vocal works), Electronics is tightly structured (carefully planned tonal aggregates, rhythmic distributions, and controlled "aleatoric" elements). The 11 sections are Curtain Music, Overture, Waltz, Trio, Postlude, Declamation, Song, Echo-Stretta, Stretta-Coda, Scherzo, and Largo.
Balanchine introduced Gassmann to the people around director Alfred Hitchcock, who at the time was editing The Birds. In addition to a richly descriptive score by Bernard Herrmann, the film needed "exceptionally strange (sounds) with which to terrify people" (Sala). Gassmann suggested that Sala's electronic devices might fill the bill and flew to Berlin with the initial film roll containing the sequence with the birds attacking the house. Sala then completed the rest of the synchronized sounds in collaboration with Gassmann.
Gassmann became a friend of Clayton Garrison, the first dean of the University of California at Irvine, and upon his death, Gassmann left a bequest of approximately 250,000 dollars to build and operate an electronic music lab bearing his name on that campus. The Gassmann Electronic Music Studio finally opened under the supervision of Christopher Dobrian in 1996.