Daughter of French-born Robert Perreau-Saussine and Eleanor Child Perreau-Saussine, she was born Ghislaine Elizabeth Marie Thérèse Perreau-Saussine. Perreau achieved success as a child actress in a number of films. She got into the business quite by accident. Her older brother Gerald was trying out for the part of the title character's son in Madame Curie. Because their mother could not find a babysitter, she took Gigi along. The two-year-old, who could speak French, got the (uncredited) part of Madame Curie's daughter Ève (while Gerald would have to wait a year to make his film debut in Passage to Marseille).