palate, roof of mouth
腭 centers on the palate, the roof of the mouth, as a specific anatomical part.
腭 combines the flesh radical ⺼ with 咢, which likely contributes the sound. The character is used for the palate, a fleshy part of the mouth.
The flesh radical ⺼ on the left points to a body part, and 咢 on the right suggests the sound. Picture the fleshy roof of your mouth as the part that resonates when you make the 'gaku' sound.
For ガク, imagine tapping the roof of your mouth with your tongue and making a 'gaku' sound: gaku -> ガク, right where the palate is.