Evidence
Scientific research has been carried out showing the benefits of not only chewing gum, but mastic gum.
Research suggests that chewing only 5 minutes a day can increase bite force significantly.
Since mastic is 10x harder than regular gum, results can be 10x faster and 10x better than chewing regular gum. Most off the shelf gums are too soft to even activate the masseter muscles (jawline muscles), rendering them ineffective to increase jaw strength and size.
Mastic Gum Studies/Articles/Books
A pilot study of the effect of masticatory muscle training in facial growth in long-face children:
https://core.ac.uk/download/pdf/85222094.pdf
https://www.myimagedental.com/blog/does-chewing-gum-help-your-jawline/
Page 43: https://books.google.co.uk/books?id=V9YaCgAAQBAJ&dq=info:V6lAHbHqP1AJ:scholar.google.com/&lr=
Mastic gum is recommended in the book 'Breath' by James Nestor
Chewing Gum Studies/Articles
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5893462/
https://www.koreascience.or.kr/article/JAKO201433752159269.page
https://news.stanford.edu/2020/07/21/toll-shrinking-jaws-human-health/
https://aaamed.org/files/ajam2019.pdf#page=44
https://www.sciencedaily.com/releases/2019/05/190531100544.htm
Page 23, Table 2: http://www.pps.org.pk/PJP/12-1/Amer.pdf