Marini was born in Grasse, Alpes-Maritimes, France, to a Greek mother and Italian Father. After working as a baker in his father's bakery since he was eight years old and high school, he joined the French army and was stationed in Paris, where he served as a fireman in the Paris Fire Brigade. It was in Paris where Marini met Fred Goudon, a Photographer who introduced him to the world of modeling. After fulfilling his military duties, he went to the United States to learn English while working as a model. He began his career as a model in his early twenties. One of his first jobs was a television commercial for Bud Light beer. He made his acting debut at the age of 29, in the 2005 horror film Screech of the Decapitated.
Gilles and his professional partner, Cheryl Burke came in as the runners-up of the American televised ballroom dancing competition, Dancing with the Stars. On May 19, 2009, then seventeen-year-old Gymnast Shawn Johnson and her partner, Mark Ballas were crowned champions of that season, defeating Marini & Burke by 1%.
He was introduced as a recurring character on ABC's family drama Brothers & Sisters. He played Luc Laurent, the French love interest of Rachel Griffiths' character Sarah Walker in what was originally supposed to be a five-episode arc, but was promoted to a series regular. The series was cancelled in May 2011, after the fifth season ended. He also appears as Bay Kennish's biological Father, Angelo Sorrento, in Switched at Birth. On September 18, 2012, it was announced that he would become a series regular for the second season.