gnaw
囓 centers on the act of gnawing: biting or chewing persistently with the teeth.
囓 combines 口 (mouth) with 齧 (gnaw, bite), reinforcing the idea of gnawing. The exact historical development is uncertain, but it likely serves as a semantic compound emphasizing the action of the mouth.
The mouth 口 on the left and the gnawing component 齧 on the right together show a mouth persistently chewing: gnawing.
For ゲツ, imagine a mouse gnawing on a piece of wood, making a repetitive 'getsu-getsu' sound: getsu -> ゲツ.