I have a decent jaw + low body fat, so I have a slightly above average jawline. Chewing gum does help me a little.
Chew gum will improve your jawline if you have medium-high bf% provided you have a decent jaw already, but it's not going to make you go from incel-tier jawline to chad-tier jawline.
twistzz was overweight and yea lets say that chewing gum helped him a bit also. But even when taking that into consideration his cheek bones have always been the way they are now in order for him to have his current jawline (high cheek bone etc) there are many youtube videos that explain all of these different factors but my point is that if your jawline is non existent/weak and your body fat % is already low then no face exercises or chewing gum will make your jawline significantly better