嘎 is onomatopoeic. 嘎嘎 imitates a duck quacking; 嘎吱 a creak under pressure; 嘎嘣 a crisp snap or crunch.
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嘎 has three readings. First-tone gā is the everyday sound word above. Second-tone gá is a cackling noise and also writes a child's spinning toy. Third-tone gǎ is a northern dialect word for naughty or ornery, and powers the internet slang 嘎了 (done for).
Three-stroke mouth radical on the left, the indexing radical. 嘎 is onomatopoeia for a sharp creak or squawk: a duck's quack, a door's creak, a sudden snap. The mouth radical signals sound-from-the-body, as in 哈, 哎, 啦.
Right side 戛 supplies the sound, jiá shifting to gā through a regular j/g alternation in colloquial readings. 戛 originally named a small halberd and by extension the tap of metal on metal, so the phonetic also brings a sonic echo that fits the onomatopoeia.
