What does that have to do with the gospel Jesus and his disciples preached? What gospel did Jesus mean here when he said "this gospel":
Matthew 24:14 And this gospel of the kingdom shall be preached in all the world for a witness unto all nations; and then shall the end come.
I don't think Paul was preaching a new gospel.
We have to remember that Jesus was preparing the Jews for the gospel that was going to be preached after His crucifixion and resurrection. Paul is not preaching another gospel. Jesus told His disciples that everything they needed to know could not be told to them at that time, but the Holy Spirit when He comes will lead them into all truth.
Jesus preached to those who were still believing in the Messiah to come. Most of these people rejected Him and His message. We see that when the crowds realised that commitment to Him was more than just getting free lunches and free healing. When He started telling them about being identified with His death, most of them felt that was too difficult for them and they departed from Him. Jesus was so concerned about that He asked his 12 disciples whether they would depart from Him as well. Peter replied, "Where will we go? You have the words of eternal life."
So what Jesus preached to the Jews was that the kingdom of God is with them in His personal presence with them, and He encouraged them to believe in Him as their Messiah. Most of the Jews, except for the 12 disciples, rejected Him and clamoured for His death in front of Pontius Pilate.
So, after His death, resurrection and ascension, the Holy Spirit came on the day of Pentecost, and Peter preached the first gospel message, "Believe on the Lord Jesus Christ and you will be saved", plus a lot of other things, and as a result 3000 turned to Christ. But neither Peter nor John had received the full revelation of justification by faith, and their ministry was to the Jews and not yet to the Gentiles, although God had to give Peter a vision to encourage him to give the gospel to Cornelius' household, with the result that the Holy Spirit fell on them too. I think this was God's preparation for when Paul came to Jerusalem to share the gospel he was preaching to the Gentiles, and Peter's testimony was a significant factor for the Apostles giving Paul the right hand of fellowship and to acknowledge that he was preaching the same gospel as they were.
When, after his conversion, Paul went into seclusion in Arabia for three years. Jesus gave him the same training in that three years he gave to the other Apostles in the three years He was with them. Along with that, He gave him the completion of the gospel which revealed justification by faith bringing peace with God.
It was not that those who received Christ under the ministry of Peter, John, Stephen and Phillip, were different to those converted under Paul's ministry, but the Holy Spirit through Paul completed the information about how converts gained full standing with God in Christ, revealing the full mystery of the gospel to Jews and Gentiles. It was not that Paul's gospel was superior or any way different than the others, but merging the teaching the Holy Spirit gave through all the Apostles, including Paul, completed the full "theology" (for want of a better word) of the gospel of Christ.
Jesus' preaching showed how the righteousness they needed had to be more than just the righteousness of the Pharisees which was just a compliance with the external rules of the Law, but the heart remaining unaffected. This is how, under Mosaic Law, a man can comply with the Law even though he lusts in his heart after a "hot chick" but keeps the Law by refraining from fornicating with her. But Jesus preached that there must be a change of heart that that just lusting after a woman in his heart falls short of the righteousness that God requires.
The completion of the gospel revelation through Paul, showed the way that a convert to Christ can achieve the righteousness that is required - by receiving the righteousness of Christ: "Christ was made sin for us, so that we can become the righteousness of God in Him". Jesus didn't preach that, because He had not died on the cross yet, but after His death and the abolition of the Old Covenant, and the coming of the Holy Spirit, the full revelation of converts to Christ becoming the righteousness of God was able to be revealed.
I hope this sort of makes it clear.