I typed and deleted this from my original comment.
My mother was walking down the street, noticed a cute old man. He comes up to her and says, "you chomp your gum like a pig!". My mother was flabergasted and said nothing. I told her later...you may have offended him unintentionally...he was rude on purpose.
I must admit, sometimes it is all I can do not to reach over and just smack someone who is chewing gum, especially if they are popping it, but I would never say anything to their face. My grandmother was a good Baptist woman from Virginia. She may not have approved of gum chewing, but she would never have approved of being rude, either.