One of the reasons there is such confusion concerning the true nature of the “gospel” is because very few times in the New Testament does anyone actually detail what the gospel is. Many times New Testament writers talk about the gospel, but very rarely does anyone actually detail the gospel itself. Why? Because the whole New Testament assumes the gospel of the Old Testament.
The New Testament assumes the same “gospel” of the Old Testament because it assumes the same cosmogenical framework, the same eschatological conclusion, and the same messianic hope of the Old Testament.