Jesus PREACHED the gospel in the books we call the NT gospels ... what did He preach? (remember the Gospel according the the writers were not yet written,)
Mat 4:23 And Jesus went about all Galilee, teaching in their synagogues, and preaching the gospel of the kingdom, and healing all manner of sickness and all manner of disease among the people.
So what gospel did jesus preach Mark???
My Jesus and my Gospel are capitalized; but to answer your question, the life, teachings, and results of Jesus the Christ. If that is not enough for you, then feel free to wander along the byways of men, as the Reformation does.