adverb, adverb taking the 'to' particle
Describes sleeping deeply and peacefully. Often used with verbs like 寝る or 眠る.
昨夜はぐっすり眠れた。
I slept soundly last night.
赤ちゃんがぐっすり寝ている。
The baby is sleeping soundly.
I was tired, so I slept soundly.
The baby has fallen into a deep sleep.