The Gospel is all the GOOD NEWS from the beginning of our journey by the cross all the way to the ultimate place of spiritual maturity in Christ. How do I know?
Hebrews 3 through 4 compared the exodus journey from Egypt through into Canaan by a GOSPEL being preached to them which corresponds to our need to believe OUR GOSPEL and go all the way into entrance in "the rest."
Hebrews 4:2-3 KJV For unto us was the gospel preached, as well as unto them: but the word preached did not profit them, not being mixed with faith in them that heard it. (3) For we which have believed do enter into rest, as he said, As I have sworn in my wrath, if they shall enter into my rest: although the works were finished from the foundation of the world.