God's message is called "the gospel of the Kingdom of God" because that is the goal that He wants us always to focus on. He wants it to come immediately to mind that we are headed toward His Kingdom. We not only have to know that it is coming, but also what we must do to prepare ourselves for it. God calls it by what it ends with, what it will produce - the aim, the result.
However, it also includes all the preparatory material that we need to get there, as well as any other information that may be helpful or provide background to those instructions. "The gospel of the Kingdom of God" is a huge umbrella, under which stands a massive amount of teaching. Each piece of knowledge contained in the gospel ultimately feeds into this central truth: that humanity can have salvation by being born into God's Family - His Kingdom - through the resurrection from the dead.
The gospel message includes topics like the nature of God, the fruit of the Spirit, the Beatitudes, Christ's sayings and word, what the Kingdom is liken to, preparing to enter into, understanding prophecy and hidden mysteries, identification of false systems, and other foundational truths. It includes the creation and God's sovereignty, providence, and intervention in its affairs.
This gospel, includes everything that is necessary to call, teach, correct, and motivate an individual so that he can be converted and inherit the Kingdom. The true gospel is the complete revelation of God to man. It includes everything needed to know, for those that are inherit eternal life.
This is the good news and something that fills you up!