The common modern understanding of the Gospel is that “Jesus died and rose again so we can go to heaven when we die”, but Luke writes several times that people proclaimed “the gospel” before Jesus even said that he was going to die and rise again (Luke 3:18, Luke 9:1-6, Luke 20:1). So, what would the hearers of John the Baptist, the Twelve, and Jesus have heard when “the gospel” was preached to them, and how does this relate to the death and resurrection of Jesus? The preexisting story of the Old Testament and the Jewish apocalyptic framework of the first century gives us a clear answer.
Check out this video on Paul’s two-fold gospel: https://youtu.be/ZILhzVZTxGs
Check out the Enduring Witness documentary: https://youtu.be/410M_gYZxvY