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).
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.