It is quite plain what the apostles preached (Acts 8:12; 19:8; 20:25; 28:23, 31). They preached the Kingdom of God with the same zeal as their Master, who had given them the example. To those who have ears to hear, it is clear that the gospel of the Kingdom is the gospel. Otherwise, why did Christ not call it something else?
Every time the word gospel appears—if Jesus qualifies the word at any point—it is always "of the Kingdom of God" or "of the Kingdom of heaven." That is what He preached! He preached the coming of a great Kingdom that would turn this world upside down and establish His Father's rule over all things.
That is what He lives for—and I use the present tense purposely. He still lives for it! He is just anxious to come back and finish His work—this time as King of kings and Lord of lords with the authority to make real changes. This is the same gospel—the same message—that His ministry must teach. We must preach the Kingdom of God.
We know that grace, peace, salvation, and Christ's life and example are certainly part of that preaching, but the primary thrust is the Kingdom of God. Our hope of being resurrected and changed to be part of that Kingdom, and all of the things that come with it, will all come about because the gospel of the Kingdom is the focus. This is how God works through human, physical, fleshly people. He gives them the gospel, and He sends them out. It must be preached, for by it salvation comes (Romans 1:16).