With a firm, strong action (e.g. to grab, hold, or shut something tightly). An onomatopoeic/mimetic adverb expressing forcefulness or solidity.
彼は落ちかけたコップをガシッと掴んだ。
He grabbed the falling glass firmly.
ドアをガシッと閉めて部屋を出た。
She slammed the door shut and left the room.
重い荷物をガシッと抱えて階段を上った。
He hoisted the heavy luggage and climbed the stairs firmly.