Spurs hold a 2-1 lead following the first match at White Hart Lane but Gallas believes that they cannot afford to protect the slender lead.
The French defender told reporters: "If we put them under pressure straight away they will be wary because if they concede a goal, they will be in trouble.
"We don't have to try to come back to London with a 0-0 scoreline. We have to play our game, the way we play all the time and then I think then we can qualify."
The winner of the tie will meet either Inter Milan or CFR Cluj in the last 16 of the competition.