How should I understand the expression "all that"?
"All that" can also be understood as "all of that." The speaker has used this expression to emphasize that Chandler chewed a large amount of quitting gum, and that all his effort to quit smoking would go to waste if he started again. "All that" here is being used as a point of emphasis. Ex: Wow, look at all that food. You made all that? Ex: Look at all that people. It's so crowded here.