You seem to have two gospels. Jesus said that the gospel he and the disciples preached ("this gospel") "shall be preached in all the world unto all nations."God was reconciling heaven and earth.
Israel was to be a holy nation of priests on earth, the earthly kingdom, to bring the Gentile nations to God. They were to believe on the name of Christ and the gospel of the kingdom. They rejected Christ and this was interrupted by the dispensation of grace. After this dispensation God will turn again to Israel and the kingdom on earth will be established for a thousand years, then the new heavens and earth and all things will be gathered in Christ.
Paul revealed a heavenly kingdom in this dispensation, the body of Christ would reconcile the heavenly kingdom. Both Jew and Gentile are to believe the gospel of grace.
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.
"This gospel" -- not a new improved version of it given to Paul.
Elsewhere he told them:
Mark 16:15 And he said unto them, Go ye into all the world, and preach the gospel to every creature.
I don't think there are versions of the Gospel. John called it "everlasting." I think the gospel message is consistent from Genesis to Revelation if we are reading the Bible correctly.
Revelation 14:6 And I saw another angel fly in the midst of heaven, having the everlasting gospel to preach unto them that dwell on the earth, and to every nation, and kindred, and tongue, and people,
God has always been a gracious God, wanting to forgive people when they repent. It is not as if Paul invented the idea of grace. Think of what God told Jonah:
Jonah 4:11 And should not I spare Nineveh, that great city, wherein are more than sixscore thousand persons that cannot discern between their right hand and their left hand; and also much cattle?