Gassmann was born in Rome, Italy, the son of Italian actor Vittorio Gassman and French actress Juliette Mayniel. He is of Jewish, German and French descent.
He debuted at 17 in the autobiographical film Di padre in figlio, written and directed with his father, under whose tutoring he later studied in the Theatre Workshop of Florence. Among his theatrical activity, he was noted for his playing in Pier Paolo Pasolini’s Affabulazione (1984). In 1996 he began an artistic partnership with his friend Gianmarco Tognazzi with whom he co-acted in popular movies like Uomini senza donne, Facciamo festa, Teste di cocco, Lovest, I miei più cari amici and Natale a Beverly Hills, in the theatrical version of Some Like It Hot and as dubbers of the Disney cartoon Eldorado. In 1997 he imposed internationally with Ferzan Özpetek’s Hamam. He was subsequently chosen by Yves Saint Laurent as testimonial for his perfume Opium, and posed in a nude calendar for the Italian magazine Max. He also became a testimonial for Lancia of Musa, as well as appearing in an ad for the Glen Grant liquor. As an actor, he worked for several TV miniseries and, in France, in the action movie Transporter 2 (opposite Jason Statham), produced by Luc Besson. In 2006, Gassmann appeared in the comedy Non prendere impegni stasera. He then took part in TV dramas such as Piccolo mondo antico, Le stagioni del cuore, La guerra è finita and La Sacra Famiglia; he also acted in the US action film Transporter 2 and in Italy in the comedy Non prendere impegni stasera. In 2008 he acted in the film Caos calmo as the brother of the main lead, interpreted by Nanni Moretti. For this role, he was honoured by many awards: the David di Donatello award for a supporting actor, the Ciak d’oro, the Nastro d’Argento and the Globo d’oro of the foreign press. In the same year, he adapted for the stage the teleplay 12 Angry Men, written by Reginald Rose in 1954 that was made into a film by Sidney Lumet in 1957: the performance was taken up again in 2009, year in which he also appeared as leading actor in 4 padri single by Paolo Monico, Ex by Fausto Brizzi and Il compleanno by Marco Filiberti as well in the TV miniseries Pinocchio directed by Alberto Sironi. He is acting Tutor in Cinecittà at Act Multimedia.
Since 1998, Gassmann has been married to actress Sabrina Knaflitz. They have a son, Leo.