jah59,
Like you, I find God's Word to be faithful and true.
In response to your opening question, there can be no doubt about what constitutes the kingdom of God, and the Gospel, so well documented are these themes.
In Ephesians 3: 1-12, Paul says, 'If ye have heard of the dispensation of the grace of God which is given me to you-ward:.......That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel:....According to the eternal purpose which he purposed in Christ Jesus our Lord: in whom we have boldness and access with confidence by faith in him.'
In other words, the good news is that Gentiles, along with Jews, are offered salvation from sin and death through faith in Jesus Christ.
This is further explained in Galatians, where it says, 'That the blessing of Abraham [by faith] might come on the Gentiles through Jesus Christ; that we might receive the promise of the Spirit through faith.'
So the good news is that we can receive the baptism of Christ ( the Holy Spirit) through faith in Jesus as the sinless Son of God.
'For as many of you as have been baptized into Christ have put on Christ.'
The kingdom of heaven, or kingdom of God, is the kingdom that a person enters when they place their faith in the king, Jesus Christ. The kingdom is INVISIBLE AT PRESENT, being the body of Christ on earth, but on the return of Christ the kingdom will have a completion on earth.
This means that the kingdom of Heaven and the Gospel have FAITH IN JESUS as a common element. The missing piece of the jigsaw is Israel. Scripture seems to indicate that the remnant of Israel have yet to face their day of repentance, when they too will take their place in the kingdom on earth. This will not be until the day of the Lord, the return of Christ to earth. 'And then shall they see the Son of man coming in a cloud with power and great glory.'