If gnash means "to grind (one's teeth) together, especially in anger," then the antonyms would be:
* Smile
* Grin
* Laugh
* Relax
* Calm
If gnash means "to strike with a sharp, grating sound," then the antonyms would be:
* Stroke
* Caress
* Tap
* Gently
* Softly
If gnash means "to make a harsh, grinding sound," then the antonyms would be:
* Whisper
* Hum
* Chirp
* Quiet
* Soft
It's important to consider the specific meaning of "gnash" in the context of your sentence to determine the most appropriate antonym.