What happened to Christ was not done in a vacuum. The events recorded in the Gospels had to have been know, otherwise the authors of the Gospels would not have been able to put them into writing, especially Luke's as he says in Luke 1...
1Many have undertaken to draw up an account of the things that have been fulfilled among us, 2just as they were handed down to us by those who from the first were eyewitnesses and servants of the word.
3Therefore, since I myself have carefully investigated everything from the beginning, it seemed good also to me to write an orderly account for you, most excellent Theophilus, 4so that you may know the certainty of the things you have been taught.
Luke 24:13 Now that same day two of them were going to a village called Emmaus, about seven miles from Jerusalem. 14They were talking with each other about everything that had happened. 15As they talked and discussed these things with each other, Jesus himself came up and walked along with them; 16but they were kept from recognizing him.
17He asked them, "What are you discussing together as you walk along?"
They stood still, their faces downcast. 18One of them, named Cleopas, asked him, "Are you only a visitor to Jerusalem and do not know the things that have happened there in these days?"
Acts 8: 1 And Saul was there, giving approval to his death. On that day a great persecution broke out against the church at Jerusalem, and all except the apostles were scattered throughout Judea and Samaria. 2Godly men buried Stephen and mourned deeply for him. 3But Saul began to destroy the church. Going from house to house, he dragged off men and women and put them in prison.
4Those who had been scattered preached the word wherever they went. 5Philip went down to a city in Samaria and proclaimed the Christthere. 6When the crowds heard Philip and saw the miraculous signs he did, they all paid close attention to what he said. 7With shrieks, evil spirits came out of many, and many paralytics and cripples were healed. 8So there was great joy in that city.
Galatians 1:13 For you have heard of my previous way of life in Judaism, how intensely I persecuted the church of God and tried to destroy it.
Galatians 1:22 I was personally unknown to the churches of Judea that are in Christ. 23They only heard the report: "The man who formerly persecuted us is now preaching the faith he once tried to destroy."
We do not know precisely who Luke received his knowledge from. Matthew and John wrote from personal experience and John Mark from personal experience and Peter.
There were a lot of false (gnostic) gospels, and the Church certainly wanted to prevent them.
My point was that Paul COULD NOT have known for certain in his lifetime that the Gospels being preached in any given location were correct. Though you are correct that the Gospel began as an ORAL rather than written tradition as Luke points out and this fact places an emphasis and tradition and disputes the medieval invention of “sola scriptura.”