God said in 5:110 :
‘O Jesus, son of Mary, remember My favour to you and to your mother,
be thankful for it; when I strengthened you with the Holy Spirit,
Gabriel, to speak to people in the cradle and in maturity and when I
taught you the Scripture, and wisdom, and the Torah, and the Gospel
(Tafsir al Jalalain)
Also mentioned in Sura Maryam, 19:29: The people asked Mary -
How can we speak to one who is in the cradle a child?
Jesus suddenly replies, in 19:30:
Indeed, I am the servant of Allah . He has given me the Scripture and made me a prophet.
So Jesus says, when he was still a baby, that God had already given him the scripture. Maybe the other three parts mentioned in 5:110, i.e. the wisdom, and the Torah, and the Gospel, were revealed in later years.
There is a gap of about 18 years in the account of Jesus in the New Testament, so they don't even know what happened to him during those years. How can they claim no revelation came to him?
Whether it was a complete revelation at once or bit-by-bit, we have no source to verify. All we know that he was supported by Gabriel during his whole lifetime.