jaw
齶 centers on the jaw, the bony structure that holds the teeth and enables chewing.
齶 combines 齒 (teeth) with 咢, which likely contributes the sound and relates to the jaw or mouth area. The exact historical development is uncertain.
The left side 齒 shows teeth, and the right side 咢 looks like a wide-open mouth with a jaw. Together they form the jaw that holds the teeth.
For ガク, imagine a jaw that can gawk open wide: gawk -> ガク, and the jaw drops in amazement.