Why does Paul spend 15 days with Kephas, making it clear he had been only with Peter and had only seen James briefly?
It was important, because of who Paul had been, a persecutor of Christians, to show that he had been converted and taught by Jesus and not by men.
It was a matter of trust, not that Paul had received a different gospel.
Read further in Galatians and you find this....
Gal. 2:2] And I went up by revelation, and communicated unto them that gospel which I preach among the Gentiles, but privately to them which were of reputation, lest by any means I should run, or had run, in vain.
Gal 2:9[9] And when James, Cephas, and John, who seemed to be pillars, perceived the grace that was given unto me, they gave to me and Barnabas the right hands of fellowship; that we should go unto the heathen, and they unto the circumcision.
Paul was making sure that he was teaching and preaching the gospel correctly, confirming with the others that he was not in error. He sought out and received the approval of his message from the others, the “leaders”, first Peter(Kephas), then the others.
Christ did not spend three years teaching and preparing the Apostles to then reveal a different gospel to Paul.
We are all one, there is no Jew, no Gentile, no man nor woman, slave nor free in Christ Jesus and there is one Christ for us all and one message of grace and salvation.
Yep, it's amazing that this has to even debated.
The second secret gospel theory is a bizarre teaching that I can hardly see being called 'Christianity' at all. 'Paulianity' or 'Darbyism' perhaps.