I am in the first pages of Michael Bird’s Evangelical Theology. He opens early on with definitions of gospel because all theology must be rooted in gospel.
He uses N.T. Wright:
The gospel is the royal announcement that the crucified and risen Jesus, who died for our sins and rose again according to the Scriptures, has been enthroned as the true Lord of the world. When the gospel is preached, God calls people to salvation, out of sheer grace, leading them to repentance and faith in Jesus Christ as the risen Lord.
Then his own definition:
The gospel is the announcement that God’s kingdom has come in the life, death, and resurrection of Jesus of Nazareth, the Lord and Messiah, in fulfillment of Israel’s Scriptures. The gospel evokes faith, repentance, and discipleship; its accompanying effects include salvation and the gifts of the Holy Spirit.
Defining gospel is necessary, and difficult!