Third-tone gǎ is northern-dialect, describing a child as cheekily clever or a person as ornery: 嘎小子. Online it powers the slang 嘎了 (kicked the bucket, game over), a euphemism for dying or failing. Informal; not used in writing.
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.
