Correct me if I'm wrong, but there are four gospels, not one, no? The first one written was Mark, around 75 CE. There was one earlier version of Jesus, namely Paul's. Of the other three models of Jesus, Matthew and Luke have similarities, but John's is closer in many ways to Paul's.
There are 4 texts but one gospel really, and the earliest was written probably in the mid 50s, 20 something years after Jesus died and the other 2 synoptics would have been written before 70AD according to the evidence and if we don't assume that prophecy is untrue and that they had to have been written after 70AD, the destruction of the Temple date. There may have been writings circulating before the mid 50s also.
If you read Acts 9:1-30 and Acts 11:19-30 you will see that Paul spent plenty of time with Christians in different places. When he was converted Jesus said to him. "Saul, why do you kick against the goad?"
It was like Saul had put thought into who Jesus was and was in denial and turned completely against the idea of Jesus being the Messiah even though it kept pricking him.
No, Paul never met Jesus. And he says
Galatians 1:11 For I would have you know, brethren, that the gospel which was preached by me is not man's gospel. 12 For I did not receive it from man, nor was I taught it, but it came through a revelation of Jesus Christ.
and what Paul claims to know about the biography of Jesus will fit into two or three lines.
Paul spent time with and learnt a lot from the early Church about Jesus. That is obvious. But it seems that he had worked out from what he already known of Jesus that he could be the Messiah and what the gospel was. He was learned in the scriptures and could see the potential prophecies about Jesus and what he had done and could see the gospel in the Hebrew scriptures. (something you still can do of course)
Paul was able to preach the gospel early upon being converted because he knew about it from the OT, it had been revealed to him.
Paul probably knew plenty about the biography of Jesus, having spent time in various churches, but he knew he did not have to tell people about that in his letters. Just because he does not write a biography in his letters does not mean that he did not know about Jesus life.
John, being a later writer of the gospel, puts in it things that had not been witnessed to in the synoptics but were still present in the early church. If you think Paul and John are similar then it is plain that what John wrote was in the earlier church while Paul was there and preaching.
Neither of them are gnostic, they are anti gnostic.
In my view that claim is not sustainable. There are five distinct versions of Jesus in the NT. Paul's and John's are influenced by gnosticism, lived in heaven with God, and created the material universe. Mark's is an ordinary Jew who doesn't become son of God till God adopts him (on the model of David in Psalm 2:7) on his baptism; and Matthew's and Luke's are the product of the divine insemination of a virgin.
That is only 3 versions. But you don't say how they are contradictory. They actually add to each other. So we get a more full picture of Jesus.
Mark does not say that Jesus became God's Son at His baptism. Mark just starts his story there at the baptism.
The impregnation of a virgin shows that God is the Father of Jesus and that He came from heaven. John makes this more clear.