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.”
Which is what he addresses in Galatians, especially chapter 1.
Galatians 1:6 I am astonished that you are so quickly deserting the one who called you by the grace of Christ and are turning to a different gospel— 7which is really no gospel at all. Evidently some people are throwing you into confusion and are trying to pervert the gospel of Christ. 8But even if we or an angel from heaven should preach a gospel other than the one we preached to you, let him be eternally condemned! 9As we have already said, so now I say again: If anybody is preaching to you a gospel other than what you accepted, let him be eternally condemned!