The Gospel of the Kingdom is the same as the Gospel that Paul preached. What church do you attend? Who teaches this?
If you are referring to Col 1:23
23If ye continue in the faith grounded and settled, and
be not moved away from the hope of the gospel, which ye have heard,
and which was preached to every creature which is under heaven; whereof I Paul am made a minister;
I doubt you will find any credible scholarly commentaries that suggest that Paul meant that everyone had already heard the gospel. He simply meant it had been sent by God to the whole world to hear.
Here are quotes from just three of the easily accessible commentaries in BibleHub.
To every creature which is under heaven - A Hebraism for the whole human race, and particularly referring to the two grand divisions of mankind, the Jews and Gentiles; to both of these the Gospel had been preached, and to each, salvation by Christ had been equally offered. And as none had been excluded from the offers of mercy, and Jesus Christ had tasted death for every man, and the Jews and Gentiles, in their great corporate capacity, had all been invited to believe the Gospel; therefore, the apostle concludes that the Gospel was preached to every creature under heaven, as being offered without restrictions or limitations to these two grand divisions of mankind, including the whole human race.
(1) That it was designed to be preached to every creature, or that the commission to make it known embraced everyone, and that, so far as the provisions of the gospel are concerned, it may be said that it was a system proclaimed to all mankind; see
Mark 16:15. If a vast army, or the inhabitants of a distant province, were in rebellion against a government, and a proclamation of pardon were issued, it would not be improper to say that it was made to every one of them, though, as a matter of fact, it might not be true that everyone in the remote parts of the army or province had actually heard of it.
1. To whom it was preached: To every creature under heaven (v. 23), that is, it was ordered to be preached to every creature, Mk. 16:15. It may be preached to every creature; for the gospel excludes none who do not exclude themselves. More or less it has been or will be preached to every nation, though many have sinned away the light of it and perhaps some have never yet enjoyed it.