Chelsea, knocked out of the Champions League's last 16 by Barcelona in midweek, took a deserved lead through Ghana midfielder Michael Essien after 14 minutes.
Midfielder Jermaine Jenas equalised just before halftime before Gallas blasted in a curling shot from 25 metres in the second minute of stoppage time for the Premier League leaders.
Victory moved Jose Mourinho's side ever closer to retaining their title. They have 75 points and an 18-point lead over Manchester United, who have two games in hand before hosting Newcastle United on Sunday.
There were also comfortable wins on Saturday for Bolton Wanderers, firmly in the Champions League race after trouncing West Ham United 4-1, and Everton who beat Fulham 3-1.
Bolton (27 games) have 45 points, four less than fourth-placed Tottenham (29) and one more than Arsenal (28).
Bottom club Sunderland's first game under caretaker manager Kevin Ball ended in a 1-0 defeat by Wigan Athletic.
Sunderland look all but doomed, Birmingham City stay in the drop zone after drawing 1-1 with West Bromwich Albion in the Midlands derby, but second from bottom Portsmouth kept their survival hopes alive with a stoppage-time winner to beat Manchester City 2-1.
DOMESTIC DOUBLE
Chelsea are still bidding for a domestic double with an FA Cup quarter-final against Newcastle on Mar. 22 and, looking back to their European exit at the Nou Camp, Chelsea captain John Terry told Sky Sports: "Everyone was disappointed - but we've all picked ourselves up in training in the week.
"After a bad week, we're buzzing and everyone's happy. We've still got a great chance of the double - we know we can do it."
Saturday's game was preceded by a minute's applause for former striker Peter Osgood, who died on Mar. 1. Former Chelsea players of the 1960s and 1970s gathered on the centre circle.
Spurs have not beaten Chelsea in the league since 1990 and the defeat risks loosening Tottenham's hold on a place in the Champions League qualifiers.
Arsenal, in sixth, will look to cut the gap which separates the two north London clubs when they face third-placed Liverpool at Highbury on Sunday.
Down in the relegation zone, an equaliser by substitute Nathan Ellington kept West Brom in 17th place on 27 points -- three clear of Birmingham.
Though his side remain in the drop zone, City manager Steve Bruce was relieved to have taken a point.
"Analysing it for us defensively - it was a shocking afternoon for us," he said.
"We just couldn't defend properly at all...we've probably had more luck this afternoon than we've had all season."
Portsmouth's luck looked to have run out after Richard Dunne's 83rd minute equaliser for Manchester City but Pedro Mendes struck a glorious second of the day to move Harry Redknapp's men on to 21 points. Sunderland are bottom on 10.
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