Why is it always "gnash one's teeth"? Is anything else ever gnashed?
The word "gnash" is specifically used only in terms of the teeth.
verb (used with object)
verb (used without object)
From my experience, I don't think this term can be utilised in any other fashion. Additionally, I have never seen it used another way. It is likely a term that evolved specifically to describe the action. It is also possibly onomatopoeia for the sound of teeth grinding together or the vocalised noise one sometimes makes while performing the action.
I've heard it used in relation to gearboxes in cars, for example a person not used to driving with a clutch might 'gnash the gears' while changing.