It's a little weird, and could possibly be a bad habit if it affects your breathing. Getting sufficient oxygen to your lungs while you work out is important, so if that is in anyway affecting that, then I would cut it out.
But honestly, you should be working out at a level where gum-chewing is not possible because your breathing too hard. Sounds like your kind of taking it easy at the gym. Push yourself if you really want to see results.
