Gassman devoted herself almost exclusively to the theater, with the exception of some sporadic but significant television appearances in comedies and scripts.

At the beginning of her career she spent three years in the Milan company Teatro Libero, directed by Luca Ronconi.

She performed in their show Orlando furioso, which toured in many European countries and in the United States before being adapted into a television series.

She also performed in Goldoni's The Liar, Georges Feydeau's The Cat in the Pocket, Svevo's Wedding Scenes, and Shakespeare's The Merchant of Venice and A Midsummer Night's Dream.

Her later shows included Miller's Down from Mount Morgan; Crouse's Life with Father; Mary Coyle Chase's Harvey; Iphigenia in Aulis and Helen (both by Euripides); and Viaggio a Venezia and La Bottega del Caffe (both by Goldoni).