This is a very hard question and one that will defy any explanation from an orthodox approach.
Paul was converted in 33 or 34 CE, based on the crucifixion occurring in 33. Other dates are suggested but 33 best fits all available facts. Before 32 CE for example, Pilate was secure enough in his position that he would not be swayed by the High Priest no matter how large a mob he gathered.
Paul then spent 3 years in Arabia, probably meaning Nabataea, just to the east.
According to Galatians, he returned, stopped by Jerusalem then claims he spent 14 years in Syria and Cilicia. Right here is a sign we have a problem, as Acts has him in Crete and what is now Turkey, contradicting what Paul himself claims. Sometime during this period in Syria and Cilicia, he claims to have received a revelation, which prompted him to go talk to the leaders in Jerusalem, maybe as early as 49 but more likely 50 or later. The exact nature of this revelation is not revealed.
To claim that it was that Gentiles should be included is to ignore the Old Testament which assures that the Gentiles will become God's people in multiple places. Likewise, the Dead Sea Scrolls affirm that Gentiles need not follow Mosaic law, so that is not what the revelation is. Whatever the revelation is, it must be well hidden to modern eyes. It would have been obvious to his listeners, but we are missing some key piece of evidence in seeking to understand this issue.
My current research has revealed though that there were two religious figures known in the 1st century who reportedly had a human mother and the supreme god as a father, but were not considered demi-gods. Both of these figures were associated with rising from the dead, women, slaves and rescuing their followers form the dark and desolate regions of the afterlife. Both also reportedly rode on a donkey at one significant point in time. The feasts for both involved bread and wine.
The worship of one had been banned by Rome in the 1st century BCE though, before Jesus was born. Also, this one who predated Jesus had been for some time linked by gentiles to the god of the Jews. If Paul's revelation was to link Jesus and this other entity to increase the popularity with the gentiles, it would also fit with his missionary journeys as this other entity had been greatly loved in the 1st century BCE in what is now Turkey and Greece.
However, I dare not say more with certainty as my research is ongoing. I wish to confirm that many, if not all, the issues Paul addressed in his letters can fit with the worship of this other entity as well as look for more positive signs that the linkage was made.