Spurs suffered a 2-1 defeat to Manchester City on Sunday which left them just one point above the Gunners ahead of their meeting at the Emirates Stadium.
And the Frenchman accepts that Andre Villas-Boas' men will need to get points on the board in order to recover quickly from defeat to the champions.
"It is the big derby. We need a good result there. We have to do something," Gallas told reporters.
|28/1||Tottenham are 28/1 with Bet365 to beat Arsenal 3-2 in the north London derby
"We could have got something from the game. We could have played better, we couldn't keep the ball very well and that was maybe the key of the game."We were very frustrated because we then had the chance to score the second goal."
With Spurs having beaten Manchester United at Old Trafford for the first time in 23 years earlier this season, Gallas believes that his team-mates are approaching every away game full of confidence.
"If we believe in it, maybe we can finish fourth, but there are so many teams who want to finish fourth as well," he added.
"All the fans, people who watch football, are very excited to know who will finish fourth.
"After what we did at Manchester United we have to believe we can win anywhere. We played very well there, now when we play away we have to be convinced we can do something and come back to London with three points."