The modern-day gospel says, "God loves you and has a wonderful plan for your life. Therefore, follow these steps, and you can be saved." Meanwhile, the biblical gospel says, "You are an enemy of God, dead in your sin, and in your present state of rebellion, you are not even ale to see that you need life, much less to cause yourself to come to life. Therefore, you are radically dependent on God to do something in your life that you could never do."
The former sells books and draws crowds. The latter saves lives.
Platt goes on to define the gospel:
The just and loving Creator of the universe has looked upon hopelessly sinful people and sent his Son, God in the flesh, to bear his wrath against sin on the cross and to show his power over sin in the Resurrection so that all who trust in him will be reconciled to God forever.