adverb, adverb taking the 'to' particle
fast asleep; snoring sound; grumbling sound
Onomatopoeic adverb describing deep sleep with snoring, or a low grumbling noise. Often used with 寝る or 眠る.
疲れていたので、ぐうぐう寝てしまった。
I was so tired that I fell fast asleep, snoring away.
隣の部屋からぐうぐうといういびきが聞こえる。
I can hear loud snoring from the next room.