It has ALWAYS been about GRACE. Anything God has ever or will ever do for us is through HIS GRACE toward us. You know that. But He has manifested His grace in different ways at different times. Adam, Abraham, Moses, David, Christ, Peter and the 11, Paul. And for different reasons, but the end result WILL BE: "That in the DISPENSATION OF THE FULNESS OF TIMES, he might gather together in ONE ALL THINGS IN CHRIST, both which are IN HEAVEN, and which ARE ON THE EARTH; even in him." Eph. 1:10.
Who would benefit from God's grace on this earth? Israel, of course. The Abrahamic COvenant and the Davidic Covenant testify to this. GOd has promised them real land on this earth. For them, forever. And the Kingdom Gospel also testified to this.
Who would benefit from God's grace in heaven? Those who were promised ALL spiritual blessings in HEAVENLY places. That would be those who are in Christ. Who are saved by grace through faith and not of themselves, by the GIFT of God, not of their works, lest any should boast. They are saved by believing that Christ died FOR THEIR SINS, was buried, and rose again the third day. THAT is the gospel of the grace of God.
One gospel has to do with the earthly inheritance promised by God. The gospel of the kingdom. From God to Israel. And THEN to the nations, with Israel as a kingdom of priests, ministering to those nations.
The other gospel has to do with a heavenly inheritance. The gospel of the grace of God. That is found from Romans through Philemon. THAT is where you will learn about that heavenly inheritance. And no place else. Because it was hid in God from before the foundation of the world, until he revealed it to Paul. Why would He have possibly revealed this to Peter and the 11? They were promised an earthly inheritance, where they will sit on twelve thrones judging the twelve tribes of Israel. That is the convenant God made with their fathers. And He doesn't lie. His grace fills all in all.
You use the term Kingdom wildly.
Usually “The Kingdom” refers to the Kingdom Of God.
The earthly kingdom is a 1000 year reign from the throne of David (Jerusalem).
There is zero evidence in the word that even one single soul will be saved during that reign. It is a demonstration, not a plan for salvation.
One must be the beneficiary of grace to be “changed” to a spiritual body that can exist in the realm of God.
Your definition of “the gospel of the grace of God” is not from scripture, it is man made.
The belief that saves is the belief that leads one to follow Yeshua in every way, not intellectual belief that he died for our sins. As Paul noted, Satan and his angels so believe.