With a loud clattering sound (especially when opening or closing something); forcefully/flinging (open).
彼は部屋のドアをガラッと開けて入ってきた。
He flung the room door open with a clatter and came in.
朝の風で窓がガラッと閉まった。
The window slammed shut with a loud rattle from the morning wind.
古い戸棚の扉は押すとガラッと動いた。
When pushed, the old cupboard door moved with a clattering sound.
Completely/dramatically; suddenly (a major change in state or appearance).
彼の態度は一言でガラッと変わった。
His attitude changed completely with a single word.
部屋の模様替えで雰囲気がガラッと明るくなった。
By redecorating the room, the atmosphere became markedly brighter.
政策の変更で市場の状況がガラッと変わった。
The market situation changed drastically due to the policy change.