Darren Cann said that he saw the ball "clearly" cross the line and did not rely on the position of goalkeeper Edwin van der Sar in making his judgement.
Hailed as making one of the best decisions of the season so far, Cann, 38, who is regarded as the country's top referee's assistant spotted that Gallas' shot was a good foot over the touchline before Van der Sar swiped it back out.
Cann told The Independent: "I clearly saw the ball cross the line. Luckily I have been involved in many high-profile games and with the experience that you build up over time you know that you can't guess. You have to be 100 per cent certain before you raise your flag. I clearly saw the ball cross the line and there was no hesitation."
Cann was criticised by Arsene Wenger for his part in Emmanuel Adebayor's red card against Chelsea in the Carling Cup final in February - and accused of telling an "absolute lie" - but this time the Arsenal manager had only praise.
"He did his job well and I have to congratulate him," Wenger said.
The assistant referee added: "I wouldn't want to deflect from Howard's [Webb] performance because I thought he gave a fantastic refereeing performance and in some way I feel a bit guilty that I have taken a bit of praise. I saw the ball cross the line and immediately raised my flag and indicated to Howard through our microphones that the ball had crossed the line.
"It's always nice in any walk of life to receive some praise and it's very much appreciated. It's really about keeping your feet on the ground. Saturday was a wonderful day and that is now gone and I have to re-focus for the next one."