Uncertainly about the fitness of Gallas, who is recovering from a calf injury, is the reason why France coach Raymond Domenech named a provisional squad of 24 players instead of the official 23.
Over the past few days, France's most reliable and experienced defender has been training at the team camp in the French Alps resort of Tignes, raising hopes he would be fit to play in the June 11-July 11 finals.
"William is our main man at the back and we need him," Clichy, who plays alongside Gallas at Arsenal, told reporters on Sunday.
"I have no doubt he will play at the World Cup," Clichy added. "He has hiked up with us in the snow and has been training with no problems. I know him well, he's strong, he won't give up and he will make the trip with us."
"From what I see, he's physically fit and he will stay with us until the end," Squillaci said.
"There is room for optimism but it's still a calf injury and we don't know what can happen," Domenech told French television channel TF1 on Sunday about the Gallas situation.
"We won't hurry, we'll take our time before finding a solution, providing we need to find one," Domenech said.
