Here is the late John Stott’s definition of the Gospel, it is typically simple and profound:
What is the Good News?
The good news is Jesus. And the good news about Jesus which we announce is that he died for our sins and was raised from death. In consequence he reigns as Lord and Saviour at God’s right hand and has authority both to command repentance and faith, and to bestow forgiveness of sins and the gift of the Spirit on all those who repent, believe and are baptised And all this is according to the Scriptures of the Old and New Testaments. It is more than that. It is precisely what is meant by “proclaiming the kingdom of God.” For in fulfilment of Scripture God’s reign has broken into the life of men through the death and resurrection of Jesus. This reign or rule of God is exercised from the throne by Jesus, who bestows salvation and requires obedience. These are the blessing and the demand of the kingdom.
John Stott – from Christian Mission in the Modern World