Petit, who like Gallas has played for Arsenal and Chelsea, told the Sunday Mirror: "I know William well. I know he is a big character, the type of guy you can depend on. That is why he was made captain.
"He is the kind of player, the type of personality that I would want alongside me if the going is tough. You can depend on him. That is my experience.
"It was, for me, an act of frustration from William last week. He had seen the terrible injury to Eduardo early in the game and maybe he could sense that Arsenal should have made the game safe after they took a 2-1 lead in the second half."
Petit went on: "He has been criticised but all he did was show that he is human, that he has emotions.
"Perhaps he could see what damage that draw and the loss of two points could do to Arsenal at the end of the season.
"He is a born winner and a leader. And he will not be affected by any negative comments. I am sure of that. He is an experienced player who is surrounded by many young players and maybe he does not want them to finish the season with nothing.
"I would say it was frustration at Birmingham, nothing more. He hasn't suddenly become a bad player or a bad character."