The shape of your jawline depends on your genes, you cannot change it by chewing gum. Chewing gum is indeed a good workout for your facial muscles but changing the structure of your jaw is not possible.
Dhanurasana, or the Bow Pose, is the ultimate pose to stretch your body. It adds flexibility to your back and strengthens the abdominal muscles.
The muscles involved in chewing are located in your neck and cheeks. Thus it is very unlikely that you will get a sharper jawline by chewing gum.
It is not possible to achieve fat loss by chewing gum. Hence it is not effective in achieving a more chiselled jawline.
Be careful! Excessive chewing of gum can result in headaches, jaw clicking and may strain your facial muscles, causing severe pain.