Gospel means “Good News.” It is an announcement of what God has done to save us, a remarkable truth that we could never learn from any other source than God’s saving Word. Because of his love, God sent his son into the world: Jesus Christ who is fully God and fully man. Jesus always lived in perfect obedience under God’s rule, yet by dying in our place he took our punishment and brought forgiveness.
The heart of the gospel is what God has done in Jesus, supremely in his death and resurrection. Period. It is not personal testimony about our repentance; it is not a few words about our faith response; it is not obedience; it is not the cultural mandate or any mandate. Repentance, faith, and obedience are of course essential, and must be rightly related in the light of Scripture, but they are not the good news.
What the gospel announces is the profound truth that God saves sinners by his working, not ours. He “justifies the ungodly” (Romans 4.5), saving them from their sin even while they yet remain sinners, declaring them as righteous, holy, and pure as his own sinless Son, Jesus, who took their place of punishment.
We often hear calls to “live the gospel” or even to “be the gospel”. In effect, this means that our own conformity to the righteousness that God demands becomes the message, rather than Christ’s life, death and resurrection. The gospel is not merely a message that people need to hear to become converted; it is the “power of God for salvation” (Romans 1.18) in every moment of the Christian life. As we mature in the Christian life, the goal is not to move beyond the gospel but to grow deeper in it, understanding more and more what it means to be part of God’s new creation: justified, sanctified, and one day glorified.
For further study, I recommend Don Carson’s essay downloadable for free. Take particular note of his section, The Gospel Is Not Just for Unbelievers, but Also for Believers, p. 24-26 of the pdf.
There is also a helpful chapter in the DeYoung/Gilbert book it’s a bit less as a Kindle version.