He began his career as a concert pianist while still a teenager, first drawing acclaim for his 1964 performance of his own piano concerto with the CBC Symphony Orchestra.
[1] In 1965 Gellman entered the Juilliard School in New York City where he studied through 1968 with such teachers as Luciano Berio, Vincent Persichetti, and Roger Sessions.
In 1973 he entered the graduate music composition program at the Conservatoire de Paris where he spent three years studying under Olivier Messiaen.
In 1978 Gellman received a commission from the French Government to compose a work for the Festival de Besançon, France, in honour of Oliver Messiaen's 70th Birthday.
The result, Deux Tapisseries for 15 players, was given its premiere on 11 September 1978, in Besançon by the Ars Nova Ensemble conducted by Marius Constant; it was given a repeat performance one month later in Paris.
Gellman's expertise in the area of orchestral music triggered a commission from the Toronto Symphony to compose Awakening in 1983, a short concert overture moving from darkness to light, chaos to order, through a gradual accumulation of energy.
Gellman's Piano Quartet (commissioned by Radio Canada) received its world premiere on 3 April 2004 in Montreal, performed by Musica Camerata.