There are some claims on the internet that chewing gum frequently for a long time (seemingly at least months) can result in a masculine-looking jaw line.
So for example according to a story told here (originally from this Chinese-language article) a woman from Nanjing, China had chewed gum every day for 6 months, and ended up with more masculine-looking jaw line.
Admittedly however there is very little evidence and above story is accompanied by rather low-quality attempts of photoshopping.
Still the same idea is expressed elsewhere. For example a question here asks:
Since teeth grinding creates hypertrophy of the masseter muscle, couldn't someone intentionally chew gum to increase the size, girth, and squareness of their jaw angle?
The question is answered by someone who portrays themselves as a "Dermatologic Surgeon" with:
The answer to your question is probably yes
It is also promoted as "the easiest and best-known ways of improving your jaw definition" in this wikiHow article.
Besides that there are websites that attempt to capitalize on the idea and promote various products that can assist in the goal towards a more well-defined jaw line. For example this one that promotes a harder gum (called Mastiha gum), this one, and this one.
Can long-term gum-chewing noticeably alter the look of the jaw line?