calyx, cup
萼 refers to the calyx, the green cup-like part of a flower that holds the petals. The 'cup' meaning is a direct extension of this botanical cup shape.
萼 combines the plant radical 艹 with 咢, which likely contributes the sound. The character is used specifically for the botanical calyx.
The top 艹 shows it's a plant part, and the bottom 咢 looks like a cup with a lid. Imagine a tiny green cup at the base of a flower holding the petals.
For ガク, picture a flower's calyx shaped like a gaku (がく, frame) that holds the petals in place: gaku -> ガク.