Jesus preached the kingdom was at hand. The kingdom on earth in which Christ and the believing remnant of Israel will reign for a thousand years. The apostles were looking for the restoration of the Israel's earthly kingdom.........[Act 1:6 KJV] "When they therefore were come together, they asked of him, saying, Lord, wilt thou at this time restore again the kingdom to Israel?"
Paul didnt preach the earthly kingdom. The kingdom of God is used in regard to God's kingdom including heaven and earth
There is one Gospel; the Gospel of Jesus Christ.
The same Gospel Jesus preached, the same Gospel Paul preached, the same Gospel that is preached today.
Therefore, King Agrippa, I was not disobedient to the heavenly vision,
but declared first to those in Damascus and in Jerusalem, and throughout all the region of Judea, and then to the Gentiles,
that they should repent, turn to God, and do works befitting repentance.
Then Peter said to them, “
Repent, and let every one of you be baptized in the name of Jesus Christ for the remission of sins; and you shall receive the gift of the Holy Spirit. Acts 2:38
Now after John was put in prison, Jesus came to Galilee, preaching the gospel of the kingdom of God, and saying, “The time is fulfilled, and the kingdom of God is at hand.
Repent, and believe in the gospel.” Mark 1:14-15
And this gospel of the kingdom will be preached in all the world as a witness to all the nations, and then the end will come. Matthew 24:14