gnash (one's teeth) and German knusen ("to chew") ought to be related with nosh and German naschen, seeing that snack in the same sense may be derived from some onomatopoeia meaning "bite". It's a non-trivial problem since these onmnomnom forms are thought to escape regular sound change. However, you may be right that silent g is inserted by mistake, but an archaic g-prefix which may be realized as /k/ is productive in Swabian dialects, which might indicate a western Yiddish variety rather than the more eastern Ashkenaz.