Gallas, who had been a doubtful starter for the World Cup because of a calf injury but played in France's opening 0-0 draw with Uruguay on Friday, left accompanied by a team physiotherapist with 20 minutes left in the 60-minute session.
GEAR:Check out France's WC range
"It's a muscle problem, nothing serious," the spokesman said without elaborating.
Cedric Carrasso, France's number three keeper, did not take part in the session at the team's base in Knysna, Western Cape, because of a pulled left thigh muscle, the spokesman said.
France staged a public session for the first time since they arrived in South Africa and had distributed 400 invitations but only around 100 supporters turned up on a rainy day in this seaside resort.
After training on a waterlogged pitch inside the boundaries of their luxury hotel, the players went straight back into their bus without stopping to sign autographs.
The 1998 champions, whose popularity has hit a low after their Euro 2008 flop and the uninspired performances that followed, have a real problem with their fans and did nothing to address it on Monday, a supporter said.
"I was at Euro 2008 and they are making the same mistakes again," said the fan, Benoit Oudin, from Auxerre. "We would like to join the party but they're closing the door on us."
France next play Mexico on Thursday in Polokwane. They will also face hosts South Africa in Group A.
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