Gum, once made from natural resin, is now almost always made from synthetic rubber. Instead of resin, oil-based polymers are used. The exact types and amounts of these polymers are trade secrets of the manufacturers, but polyethylene is one of the common ingredients.
Thus, chewing gum, which is not intended to be swallowed, is essentially "chewing plastic." Gum spat out on the street contributes to plastic pollution and the plastic soup in our oceans because its ingredients do not break down. Unlike other plastics, gum cannot be recycled. The only proper way to dispose of it is to throw it in the general waste.
In 2014, the Dutch National Institute for Public Health and the Environment reported that "chewing gum is frequently found as litter on the streets, and emissions of microplastics can occur in this way." This observation has never led to any policy measures to mitigate this source of microplastics. Instead, sidewalks are cleaned at high costs with pressure washing to remove gum stains.
Fortunately, you can now buy plastic-free gum. Some brands have replaced synthetic gum with natural gum. So, look out for the label "plastic-free"!
