An onomatopoeic adverb expressing a short, sharp clack or snap when hard objects hit or fit together; can also indicate something closing or fastening tightly and firmly.
箱のふたがガチッと音を立てて閉まった。
The lid of the box snapped shut with a clack.
ネジをガチッと締めてから機械を動かしてください。
Tighten the screw firmly with a click before operating the machine.
ドアが風でガチッと閉まってしまった。
The door slammed shut with a clack because of the wind.