He was trained under Sita Riddez, François Rozet, Henri Norbert, Marcel Chabrier and particularly Eleanor Stuart, making his debut in 1945 with Pierre Dagenais's company L'Équipe in Shakespeare's A Midsummer Night's Dream. Later, he took part in several other performances with them including Les Fiancés du Havre by Armand Salacrou (1946). In 1949, he performed the title role in Racine's Britannicus for Les Compagnons de Saint-Laurent (his sister played Agrippine). Monique Lepage and Jacques Létourneau's Théâtre Club entrusted him with many prime roles starting with J.B. Priestley's Dangerous Corner (1954), and notably in Marcel Dubé's Le Barrage (1955), Shakespeare's Twelfth Night (1956) and Topaze by Marcel Pagnol (1957). He also played the Father in Pirandello's Six personnages en quête d'auteur, directed by Paul Hébert.
During the fifties and sixties, he influenced popular imagination with his character Capitaine Aubert in Guy Dufresne's television show Cap-aux-sorciers, broadcast worldwide. He also appeared in several films including Otto Preminger's The 13th Letter (1950), Alfred Hitchcock's I Confess (1953), Jean-Claude Lord's Bingo (1973), and Denys Arcand's Jésus de Montréal (1989). Between 1962 and 1964, he travelled to France for the series Chevalier Tempête and Les Compagnons de Jéhu, based on a work by Alexandre Dumas. In 1966, he appeared on Broadway with Geraldine Page in the comedy P.S. I Love You.
During his career, Gilles Pelletier played in many television dramas, notably Dostoyevsky's The Brothers Karamazov, Racine's Bérénice, Chekhov's Uncle Vanya, Strindberg's The Father and The Dance of Death, and in the premier of the celebrated Un Simple soldat by Marcel Dubé in 1957.
In 1964, with Françoise Graton and Georges Groulx, he founded the Nouvelle compagnie théâtrale (NCT) whose mission was to introduce students "to major universal dramatic works." There he directed several works including Anton Chekhov's The Seagull, Molière's Don Juan and Eugène Ionesco's Le Roi se meurt. In addition, he played many leading roles: Sophocles' Philoctète, Shakespeare's Macbeth, Corneille's Le Cid and Edmond Rostand's Cyrano de Bergerac.
In 1982, Gilles Pelletier left the administration of the NCT (which became the THÉÂTRE DENISE-PELLETIER in 1998). The recipient of many prizes and honours (Order of Canada 1988; Ordre du Québec, 1993; Prix Denise-Pelletier, 1998) he continuously pursued his acting career, often with young companies.