On the day when Stamford Bridge remembered Chelsea legend Peter Osgood, it was fitting that the winning goal was of such class that it could easily have come from the magical right foot of the former Blues striker.
Gallas' effort in the second minute of added-time was a wonderful curling right footer from the edge of the Spurs area, which Paul Robinson had absolutely no chance of getting to.
It was the decisive strike in a 2-1 win which saw Chelsea take the lead only to be pegged back on the stroke of half-time from Tottenham's first effort on goal.
It's now 16years and 32 league games since Chelsea last tasted defeat at the hands of their London rivals.
With the Stamford Bridge pitch finally re-laid Chelsea and Spurs set about playing controlled football in the early part of the game.
The hosts had the first chance after two minutes when Hernan Crespo flashed a low drive across the six-yard box but in-running Joe Cole was just too late.
Spurs settled quickly and Mido could have wrong-footed John Terry but chose to pass the ball instead of turning and shooting.
After 10 minutes, Shaun Wright-Phillips, free on the left, arrowed into the area and produced a fine drive which England goalkeeper Robinson did well to parry. The ball fell to Michael Essien just inside the area, but he toe-poked it wide of the unguarded net.
The Ghanaian made up for it four minutes later by scoring the opening goal.
Michael Carrick's careless pass was intercepted by Wright-Phillips on the right side of the box and he cut the ball back across the six –yard box. Crespo dummied it and left Essien with the straightforward task of stroking it past the helpless Robinson.
It was Essien's first goal for Chelsea.
Terry could have doubled the lead a minute later, but he uncharacteristically produced a weak shot which fell into Robinson's arms.
Robinson then did well on 24 minutes to stop a vicious Robert Huth shot from 30 yards, which deflected off Ledley King on its way towards goal.
With seconds remaining before the break Michael Carrick lofted a left sided free-kick to the edge of the area.
Michael Dawson out jumped Huth and nodded the ball into the box, and Jermaine Jenas reacted quickest and prodded it past Petr Cech for the equaliser.
Robinson was the first keeper in action in the second half when he held a fine Cole shot from the edge of the area.
Gallas made a determined foray from left-back and his curling left-foot shot brought another smart save from Robinson.
On 57 minutes, following a sliced King clearance which was fortunate not to end up in his own goal, Robinson made a point-blank save from Terry's powerful header.
Jose Mourinho rang the changes with 20 minutes to go, introducing Didier Drogba for Wright-Phillips and Damien Duff for Cole.
With Chelsea re-adjusting to the changes, the visitors enjoyed a period of possession.
It should have led to a second goal in the 78th minute when Jenas latched on to King's fine 60-yard pass and surged into the area, but the England midfielder's weak shot was easily smothered by Cech.
With two minutes remaining Drogba collected an angled ball from Claude Makelele on his chest after Dawson had misjudged the flight, before letting fly with a powerful volley which Robinson touched on to the post.
But it was not enough to save Tottenham, as heir was still time for Gallas to produce his magical winner.