crocodile, alligator
鱷 centers on large aquatic reptiles: crocodiles and alligators.
鱷 combines 魚 (fish) with 噩, which likely contributes the sound. The character is used for crocodilians, reflecting their aquatic nature.
The left side 魚 means fish, and the right side 噩 looks like a fierce creature with many mouths. Picture a fish-like reptile with a terrifying jaw: a crocodile or alligator.
For ガク, imagine a crocodile snapping its jaws shut with a loud 'gaku' sound: gaku -> ガク.