Gaillard began his career with Lens in 1996 joining the club's youth academy.
[1] He finally made his professional debut during the 2008–09 season coming on as a substitute in a league match against Angers in the 81st minute.
[2] Due to injuries to centre backs Eric Chelle, Romain Sartre, and Alaeddine Yahia, he made his first career start the following week playing the full 90 minutes in a 0–1 loss to AC Ajaccio.
A rarity for a youth player, Frédéric took full responsibility for the loss as it was his gaffe that resulted in Ajaccio's goal.
[3] After eleven years with RC Lens left in 2009 and, in June, joined LB Châteauroux for a trial, but he was not signed.