Gum chewing is one of those habits that not only can cause some muscles to build up but can also create wrinkles. I feel that many of the lower face wrinkles are directly caused by the habit of gum chewing. It makes sense to try to stop it and perhaps just try mints, which don't cause any issues. Good luck!
While stopping your gum chewing habit will help to prevent further enlargement of the masseter muscle, it is unlikely to cause significant atrophy. The masseter muscles are used every day for eating and talking, so significant atrophy is unlikely. A combination of eliminating gum chewing and using Botox can help relax the masseter muscle, thereby relieving pain and reducing hypertrophy of the muscle. It helps to create a slimmer, softer jawline, and eventually you may be able to retain the results without needing further Botox. I suggest a consultation with a board-certified dermatologist to discuss a long-term plan.