calyx, cup
蕚 centers on the protective outer part of a flower: the calyx, the green cup-like structure that holds the petals. The cup sense is a natural extension of that enclosing shape.
蕚 is a variant form of 萼. Both combine the grass radical 艹 with a phonetic component: 蕚 uses 品 over 亏, while 萼 uses 咢. The character originally referred to the calyx of a flower.
The grass radical 艹 sits above a shape that looks like stacked mouths 品 over a curved base 亏, like a cup holding something. Picture a green cup made of grass holding the flower petals: that's the calyx.
For ガク, imagine a gak (invented sound like 'gawk') of surprise when you see the calyx cup open: gak -> ガク, and the flower's green cup unfolds.