The gospel preached to Adam and Eve, about a coming savior that would put to death the one who brought the deception and the source of hostility
This savior would defeat the serpent and his offspring (the wicked demons and those who follow after them). And he ultimately would restore what they had lost, access to the source of life (fellowship with God).
Jesus preached about His death and resurrection, before it happened
First to the Jews, since they had the Scriptures which gave the revelation. Then Christ came to remind them and say He was the fulfillment of those promises to save a people