1an adverb meaning to turn around, go around or rotate in a circle (onomatopoeic/mimetic).
彼は振り返ってぐるっと周りを見渡した。
He turned around and glanced all the way around.
箱をぐるっと回して状態を確かめた。
I turned the box around to check its condition.
2an adverb meaning to surround or encircle (to go all the way around something).
古い城壁が町をぐるっと囲んでいた。
The old walls encircled the town.
地図で山道をぐるりと確認した。
I checked the mountain trail around on the map.