The Gospel of The Kingdom of Heaven was taught 3+ years prior to his arrest for teaching it.,,,,,,,.
By saying the 'Gospel of The Kingdom of Heaven' sounds as if you are speaking about going to the Kingdom.
Jesus taught for God's kingdom to come ( thy kingdom come..... Luke 4:43 )
In other words, instead of praying to be 'taken up' to the kingdom, or 'taken away' to the kingdom Jesus instructed to pray for God's Kingdom to come, and God's Will be done right here on Earth as it is done in Heaven.
So, the Gospel or Good News of God's Kingdom is that God's Kingdom Government to come and govern over Earth.
This coming Kingdom would be coming after a long line of earthly political kingdoms or governments - Daniel 2:44.
In other words, 'the end game' is now at hand, and the time for Jesus to come (Rev. 22:20) is near at hand.