To be startled or give a sudden, sharp surprise; a mimetic word often used as 'ぎょっとする' to describe a startled reaction.
背後から声をかけられて、ぎょっとした。
Someone spoke from behind me and I was startled.
暗い道で物音がして、ぎょっとして立ち止まった。
There was a sound on the dark road, and I froze in surprise.
彼の表情を見て周りの人がぎょっとして静かになった。
Seeing his expression, people around were taken aback and fell silent.