yeast, leaven for making liquors
櫱 centers on a fermenting agent used in brewing: yeast or leaven that starts the process of making alcoholic liquors.
櫱 is a rare character whose exact historical development is uncertain. The modern form visually combines 辥 (a complex component possibly related to crime or punishment) with 木 (tree/wood), but the connection to yeast or leaven is not clearly established.
The top 辥 looks like a tangled mass of ingredients, and the bottom 木 suggests a wooden vat. Picture a wooden fermentation tub filled with bubbling yeast starter for brewing.
For ゲツ, imagine a brewer getting the yeast ready: 'get' the leaven -> ゲツ, as you get the fermentation going in the wooden vat.