The act of ones jaw vibrating, clenching or protruding. This happens whilst under the influence of various drugs such as MDMA, Ecstasy, Speed etc. The morning after gurning all evening is very painful, and this is how people ruin their teeth on drugs.
The person who is under the influence usually chews on gum to stop themselves clenching or to stop their constant chomping on their bare teeth.
Too much gurning can lock-up your jaw in which it cannot move at all.