According to Wikipedia:

The Good News In Christianity, the gospel or evangelium (also translated as "good news", "glad tidings" and variants) is the message of Jesus, the Christ (the Messiah), specifically his atoning death on the cross and resurrection, the descent of the Holy Spirit at Pentecost as "helper" (paraclete), and the resulting promise and hope of salvation for the faithful.

Good News is the English translation of the Koine Greek ευαγγέλιον (euangelion). The Greek term was Latinized as evangelium, and translated into Latin as bona annuntiatio. In Old English, it was translated as gōdspel (gōd "good" + spel "news").

The Old English term was retained as gospel in Middle English Bible translations and hence remains in use also in Modern English. The written accounts of the life and teaching of Jesus are also generally known as "Gospels". Evangelism is the spreading of the evangelium, i.e. Christian proselytization .  . .

. . . The Christian message of Good News is described in the Bible. It relates to the saving acts of God, centred upon the person of Jesus and his substitutionary death on the cross and resurrection from the dead. Its context is the storyline of the Christian Bible as a whole, which tells of the creation of humanity, humanity's rebellion against God, and how people from all nations are restored to relationship with God through the person of Jesus. A key theme of the Christian Good News is that God offers a new life and forgiveness through Jesus. Jesus' teaching of the Good News also relates to the coming of the Kingdom of God (Mark 1:14-15).