Cech came out to meet a corner but failed to collect the cross allowing former Blues centre-half Gallas to nod home the goal which returned the Gunners to the top of the Barclays Premier League following Manchester United's win at Liverpool.
The Frenchman told Sky Sports 1: "I was really surprised to see the ball come onto my head but sometimes you have to be lucky.
"Today I was a little bit lucky as Petr is usually a good, good goalkeeper.
"But it is more important to win and I feel the boys did need to win today against a good team. Everybody knows Chelsea are very strong.
"We know we now have six points (advantage over Chelsea) and that is very important."