Agatasatru said: 'When this man was thus asleep, where was then the person (purusha), the intelligent? and from whence did he thus come back?' Gargya did not know this?

In plain words: Agatasatru asks Gargya where the man's conscious self was while he was asleep, and where it came back from when he woke up. Gargya does not know the answer.