Centre-back Gallas, who had been a doubtful starter for the World Cup because of a sore calf muscle, reassured coach Raymond Domenech by heading home a Yoann Gourcuff free-kick on 63 minutes to save the ex-world and European champions from defeat.
GEAR:Check out France's WC range
"I'm not worried at all about William's calf and that's good news," said Domenech, who will be replaced by Laurent Blanc after the World Cup and needs a strong campaign in South Africa to end a controversial spell on a high note.
Tunisia fought bravely, opening the scoring on five minutes through forward Issam Jemaa, but France, still needing time to adapt to the new, 4-3-3 system they had introduced in a 2-1 win over Costa Rica on Wednesday, improved gradually.
As against Costa-Rica, forward Thierry Henry started on the bench, leaving his armband to left-back Patrice Evra, and came on for the second half.
The only member in the France squad to have won the 1998 World Cup, although he did not play in the final, the 32-year-old Henry has had a difficult season with Barcelona and is now in the twilight of his career.
Still unfamiliar with their new system, with just one holding midfielder instead of the two in their usual, more defensive 4-2-3-1 formation, France looked shaky at the back at times.
MIDFIELD BATTLE
But, gaining confidence as the match went on and winning the midfield battle, they threatened with a low drive from the edge of the box by Franck Ribery that went just wide on 23 minutes and then with a Florent Malouda volley just before half time.
France, who made an early exit from Euro 2008 and have kept frustrating their fans with uninspired performances since, still have work to do if they are to make an impact at the World Cup.
"We played a tough international against a strong Tunisia side who caused us a lot of problems," Domenech told a news conference. "We again suffered in the first few minutes and that's something we need to work on."
Playing before a partisan, 55,000 crowd on a mild, damp evening just outside Tunis, France did try to move the ball forward and shelve the cautious, dull tactics that have been their trademark over the past few years.
"We found it hard to step up the pace but that's also because our opponents tonight did not let us," Domenech said. "The team confirmed tonight their will to play attacking football and corrected some of the mistakes at the back we saw against Costa Rica."
France will now fly to the French Indian Ocean island of Reunion to play China there on Friday in their final warm-up for the June 11-July 11 finals in South Africa, where they will face the hosts, Mexico and Uruguay in Group A.
"Warm-up games don't count," Domenech sai
