The "gospel of the kingdom" is the presentation of good news based on Christ’s glory as king. It announces the coming of the kingdom of Christ, and demands that men repent because the Messiah is coming. Everyone is to get ready; i.e. "prepare the way before him". This gospel began to be preached in the days of John the Baptist, and will be preached up to the establishment of the kingdom of Christ in manifest power (Psa. 95-96).
The Gospel of the Kingdom.
Do Christians preach the gospel of the kingdom today? Yes, and no. Yes, because we still preach the coming kingdom of Christ, and the wrath of God poured out on this world prior to Christ’s appearing, and the need for sinners to flee from the wrath to come. No, because the theme of the Christian gospel rises higher than that which John the Baptist preached. In addition to a future Messiah on earth, we preach a glorified Christ in heaven! To put it another way, there are elements of the Christian gospel message that are the same as the gospel of the kingdom, but the theme is higher!