The 'Kingdom Gospel' was preached by Christ and His Disciples during His earthly ministry.' When Paul was met by the Lord Jesus Christ on the road to Damascus, and shortly thereafter, was sent to preach the "Gospel of the grace of God" (Acts 20:24 "But none of these things move me, neither count I my life dear unto myself, so that I might finish my course with joy, and the ministry, which I have received of the Lord Jesus, to testify the gospel of the grace of God.")
first to the Jew (the Jews rejected that Gospel) and then, to the Gentiles (Romans 1:16 "For I am not ashamed of the gospel of Christ: for it is the power of God unto salvation to every one that believeth; to the Jew first, and also to the Greek."
Acts 13:46 "Then Paul and Barnabas waxed bold, and said, It was necessary that the word of God should first have been spoken to you: but seeing ye put it from you, and judge yourselves unworthy of everlasting life, lo, we turn to the Gentiles." Paul was considered to be, The Apostle to the Gentiles. He and his followers were preaching 'The Gospel of the grace of God' and NOT The Kingdom Gospel that Peter and the rest were preaching. Galations 2:7 states: "But contrariwise, when they saw that the gospel of the uncircumcision was committed unto me, as the gospel of the circumcision was unto Peter;" So, we see by this verse of Scripture, there were TWO different Gospels being preached at that time. Today, 'The Gospel of the grace of God' is preached to both Jew and Gentile alike. There is ONLY one Gospel for today.