gnaw, nibble, munch, have a smattering of
齧 centers on biting or chewing in small, repeated motions: gnawing, nibbling, or munching. The sense of having a smattering of something extends this idea to taking in only small bits of knowledge.
齧 combines 齒 (teeth) with 㓞, which likely contributes the sound and may suggest cutting or engraving. The character expresses gnawing or biting with the teeth.
The bottom is 齒 (teeth), and the top 㓞 looks like a tool cutting into something. Picture teeth working like a small chisel, nibbling away bit by bit.
For ゲツ, imagine a mouse getting its teeth into a piece of cheese and gnawing: gets -> ゲツ, as the mouse gets to nibbling.