Actually, Paul's ministry to the Gentiles did not begin until about, upwards of about three years after Paul's conversion. He was not trained by the other 12, but by Christ Himself. R. C. Sproul made some interesting observations since he studied, in great detail, the timeframes within the scriptures that are not so readily apparent:
"Paul begins Galatians by saying that he was sent by Christ and the Father, and that the other church leaders agreed with him. In
Galatians 1:11–2:10, he expands on these two points.
He begins in verses 11 and 12 by saying that the Gospel he preached was not devised by human wisdom, nor had he received it from the other apostles; rather, he was taught it directly by Jesus Christ Himself. He continues by reminding them that he had been trained in and was fanatically devoted to the erroneous traditions of Judaism. When God called Paul on the Damascus road, he was commissioned to preach the Gospel of Christ.
Paul said that when Christ called him, he did not go to Jerusalem to receive instruction from the apostles. Rather, he retired into Arabia for a time and not until three years later did he go to Jerusalem. Even then, the only apostle he met was Peter, and the only other leader he met was James, the presiding elder of the Jerusalem church." (
https://www.ligonier.org/learn/devotionals/the-preparation-of-paul)
If your understanding of Jesus' command for them to go to all the world and preach the Gospel, and that the preaching mentioned was them preaching to Gentiles, then your understanding doesn't seem to match that of the reality that Peter, James and the others were still in Jerusalem years after the ascension of Christ.
Thoughts?
MM