Drugs like MDMA are well-known for their jaw-clench-inducing effects. Take a walk around the smoking area of your local 'underground' dance club and keep an eye out for stray jaws swinging left and right. But can cannabis make you gurn? There is no scientific evidence for this effect. Some reasons you may experience gurning-like effects after consuming weed are as follows.
Some strains of cannabis are notorious for causing dry-mouth. A side effect of dry-mouth can be subconscious chewing on gums, which is easily confused with gurning. Combat dry-mouth with a drink like milk or just water, but preferably not an alcohol-based drink that will make you more dehydrated.
Another reason you may be experiencing a gurning effect is the munchies. Some people have reported feeling like they want to eat/chew something but with no food available, end up just chewing their own gums. Avoid strains which cause such intense munchies.