You see: You gnash your teeth. Others see: <Person> gnashes <his/her> teeth.
GNASH <self> You see: You grind your teeth. Others see: <Person> grinds <his/her> teeth.
GNASH <person> You see: You gnash your teeth at <Person>. Target sees: <Person> gnashes <his/her> teeth at you. Others see: <Person1> gnashes <his/her> teeth at <Person2>.