He played Stanley Kowalski in Tennessee Williams' Un tram che si chiama desiderio (A Streetcar Named Desire), as well as in Come vi piace (As You Like It) by Shakespeare and Oreste (by Vittorio Alfieri).

With Luigi Squarzina in 1952 he co-founded and co-directed the Teatro d'Arte Italiano, producing the first complete version of Hamlet in Italy, followed by rare works such as Seneca's Thyestes and Aeschylus's The Persians.

Famous movies featuring Gassman include: Il sorpasso (1962), La Grande Guerra (1962), I mostri (1963), L'Armata Brancaleone (1966), Profumo di donna (1974) and C'eravamo tanto amati (1974).

Gassman brought this production to half a million spectators, crossing Italy with his Teatro Popolare Itinerante (a newer edition of the famous Carro di Tespi).

With his natural charisma and his fluency in English he scored a number of roles in Hollywood, including Rhapsody with Elizabeth Taylor and The Glass Wall before returning to Italy and the theatre.

In the 1990s he took part in the popular Italian Rai 3 TV show Tunnel in which he very formally and "seriously"' recited documents such as utility bills, yellow pages and similar trivial texts, such as washing instructions for a wool sweater or cookies ingredients.

[10] Gassman's voice was redubbed in several of his films by historical Italian actors and dubbers which include Emilio Cigoli, Sandro Ruffini, Gualtiero De Angelis, Stefano Sibaldi, Enrico Maria Salerno and Pino Locchi.