Verses with the word gnash in the Old Testament (2 verses):

Psalms 112:10
The wicked shall see it, and be grieved; he shall gnash with his teeth, and melt away: the desire of the wicked shall perish.

Lamentations 2:16
All thine enemies have opened their mouth against thee: they hiss and gnash the teeth: they say, We have swallowed her up: certainly this is the day that we looked for; we have found, we have seen it.