"chewing a lot of gum for many hours" = equivalent to lifting 5 lb dumbbells for many hours. Is it going to help? just a little bit.
Here's what you do:
Chew one gum for many hours for a couple of days. The jaw will hurt a bit, but you'll get used to it.
Then chew two gums at the same time for many hours.
The next day, chew THREE gums.
Eventually, when you have like 10 gums in your mouth (and ffs don't swallow it) it'll be extremely hard and your jaw will hurt, but you will get some masculine jaw muscles