An onomatopoeia for a thud or a hard bump — the sound or impact when something hits something hard (often used with the particle 'to').
ドアが風でゴツンと閉まった。
The door slammed shut with a thud from the wind.
彼は机の角に足をぶつけてゴツンと言った。
He hit his foot on the corner of the desk and went 'thud'.
荷物がゴツンと床に落ちた。
The luggage dropped onto the floor with a thump.
漫画では敵に殴られてゴツンと倒れる場面が描かれる。
In manga, scenes often show someone getting hit and falling over with a 'thud'.