Gospel of the kingdom; is the gospel of the soul, which means the harvesting of the elects of God,Romans 8:19 For the earnest expectation of the creature waiteth for the manifestation of the sons of God.
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.
2, it is the restoration of the church, to the standard of the word of God, John 1:1 In the beginning was the Word, and the Word was with God, and the Word was God. 1:4 In him was life; and the life was the light of men. 1:13 Which were born, not of blood, nor of the will of the flesh, nor of the will of man, but of God.
3, the gospel of the kingdom, is all about Holiness- righteousness- and purity, Matthew 6:33 But seek ye first the kingdom of God, and his righteousness; and all these things shall be added unto you. Matthew 5:8 Blessed are the pure in heart: for they shall see God. God bless you.

The gospel of the kingdom is simply the coming of the reign of messaiah. All of the teachings and parables are about how to live our lives in to show messaiah has come.
However jesus (the messaiah) gave his life so his message of the kingdom would continue to be heard all around the world so men could live as though the complete rule of God on earth has already begun.