Gigi was born in Rome to Romano Proietti, originally from Umbria, and Giovanna Ceci, a housewife. During his youth he was keen on singing and on playing the guitar, piano, accordion and double bass in several Roman nightclubs. He enrolled in the Faculty of Law at the La Sapienza University, where he attended the mimicry courses of the University Theatre Centre held by Giancarlo Cobelli, who immediately noticed his talent as a musician and booked him for an avant-garde play.

After several stage works, in 1966 Proietti debuted both in cinema, in Pleasant Nights, and on television, in the TV series, I Grandi C

amaleonti. His first personal success came in 1971, when he replaced Domenico Modugno in the stage musical Alleluja brava gente by Garinei & Giovannini, starring alongside Renato Rascel.In 1974, after playing the role of Neri Chiaramantesi in the drama La Cena Delle Beffe, alongside Carmelo Bene and Vittorio Gassman, in 1976 started a fruitful collaboration with playwright Roberto Lerici, with whom he wrote and directed his stage plays, starting with the one-man show A me gli occhi, please (Give me your eyes, please, 1976, reported on the scene in 1993, 1996 and 2000, in a memorable performance at the Olympic Stadium in his hometown). Initially planned to be performed 6 times, the show exceeded 300 performances, with an average audience of 2,000 per performance.

He took part in several international movies, including The Appointment (1969), directed by Sidney Lumet, A Wedding (1978), directed by Robert Altman, and Who Is Killing the Great Chefs of Europe? (1978), directed by Ted Kotcheff.

Proietti was also a voice dubber of films and television shows in the Italian language. He has dubbed the voices of actors such as Robert De Niro, Sylvester Stallone, Richard Burton, Richard Harris, Dustin Hoffman, Paul Newman, Charlton Heston and Marlon Brando. His credits also include the role of the Genie in the Italian version of the Aladdin film series and Draco in Dragonheart. He also provided the Italian voice of Gandalf in The Hobbit film series, replacing the late Gianni Musy, who dubbed Gandalf in The Lord of the Rings, as well as Sylvester from Looney Tunes during the 1960s.