jaw, chin, gill
顎 centers on the lower facial structure: the jaw and chin. The gill sense is a biological extension where a similar hinged or protruding structure appears in fish.
顎 combines 咢 (which likely contributes the sound) with 頁 (head), pointing to a part of the head related to the jaw or chin.
The left side 咢 looks like a mouth with a strong jawline, and the right side 頁 means head. Together they show the jaw and chin on the head.
For ガク, imagine a strong jaw clenching with a hard 'gak' sound: gak -> ガク, as the jaw snaps shut.
jaw; chin
upper jaw; palate
jawbone; mandible
beard (esp. on the chin); goatee
cleft chin
maxillary prognathism
maxilla; upper jawbone
lower jaw; mandible
mandible; lower jawbone
double chin
chaetognath (any worm of phylum Chaetognatha)
Chaetognatha (phylum of arrow worms)
agnathans (jawless fishes of superclass Agnatha)
prognathism
jaw
to have a protruding chin; to have a turned-up chin
(person) with a turned-up chin
jawbone; mandible
to jerk one's chin; to nod toward; to point one's chin at
suddenly lifting someone's face by their chin (in a domineering yet romantic fashion)