To be honest, I can't really picture what it looks like for someone to gnash their teeth. It's the "g" in "gnash" that throws me off, possibly, but when I saw the phrase "literally gnashing her teeth" in a book recently, I tried to approximate what it would feel/look/be like. Is it biting? grinding? chewing? A forward-and-backward movement, or just side-to-side?
It was only after 48 seconds of this attempt to literally gnash that I remembered I was on the F train. I got off at the next stop and went to another car, so that no one would have to step away from me, terrified of my angry teething.