"The law and the prophets"—referring to Old Testament Scriptures—were preached until John the Baptist began his special ministry. After John's mission was over, Jesus began proclaiming the gospel of the Kingdom of God.
Christ's gospel is the prophetic proclamation of the coming world government to rule all nations and bring peace and joy to today's confused, chaotic, war-weary earth! It is the vital, dynamic, powerful, living message of God's government, first over individuals in God's church who have voluntarily come under that government, then later over all nations in the future Kingdom of God!
Of course, the message from God includes the knowledge about our Savior, High Priest, and coming King! Of course, it includes the true way of salvation, which the churches of this world do not understand! It also includes knowledge of the location of the territory to be ruled over by the King of the coming Kingdom of God—the fact it will be on this earth, not in heaven!
But there can be no government without laws, and so the true gospel also must proclaim the law of God, which will bring peace and everything good to those living in the age to come, and success, happiness and joy to the individual who keeps God's law now.
The true gospel has to do with the nations of today's world, world conditions today, and Christ's reign over all nations in the World Tomorrow! It is a "full," complete, dynamic, and powerful gospel which has been kept from the world for by the god of this world—Satan the devil! (Revelation 12:9.)