A useful exercise is to put ourselves as original readers of the gospel narratives and ask ourselves what message those writers were trying to convey. The reasoning is that anything that really counts as being fundamental to Christianity or should be considered part of the "gospel message" should be found clearly in each gospel narrative since the point of the narratives were to either spread the news about Christianity or clarify a sound message from a false one in the early church.
In other words, what items are clearly presented and obvious from any reading of each Gospel?
A list that comes to my mind:
i) Jesus was raised from the dead [the fundamental belief of all biblical Christianity]
ii) Jesus has authority given by God and elevated to God's right hand in power. (indeed Psalm 110 is supposed to be the most often quoted OT passage in the NT.)
iii) Jesus was sent by God
iv) Jesus was anointed with the Holy Spirit at His baptism [one of the very few items described explicitly in all four Gospels.]
v) The existence of Hell and importance of repentance to avoid it. (Do all the gospels describe that Jesus will be the Judge? Matthew, Luke and John do...not sure about Mark.)
vi) Jesus will return.
vii) Jesus has the power to forgive sins.
I think everyone could agree that each of these:
A. Appears and is emphasized by every gospel
B. Is made absolutely clear in every gospel without any interpretation. (or any interpretation that is needed is aided by the gospel writer, Like John 2:21.)
Are there other items that are made absolutely clear in all four gospels that any reader would come away from?