Born in Marseille, Gavanon signed a professional contract with Olympique de Marseille in the summer of 2003, and made his first start for the club in a UEFA Champions League group match against FK Partizan, which resulted in a 1–1 draw and a place in the UEFA Cup. He also appeared in the 2004 UEFA Cup Final against Valencia as a substitute, after Barthez was sent off. He made only 10 league appearances in two years, however, and with the signing of Fabien Barthez, the club decided to loan out Gavanon to Clermont to gain some first-team experience. He made 22 league appearances for Clermont during the 2005–06 season.