Basically, this:
That, anyway, is the historic definition of the gospel as it has been used in the church. And I believe it is a correct and helpful was to use the term.
However, the New Testament use of the term has a slightly different nuance. I can't stand etymological definitions, but I'm going to do it anyway:
Gospel is the Old English equivalent of "God's work," it was used to translate the Greek term euangelion, or "good news" (eu = good, angelion = message, like angel). The term was primarily used in the ancient world to bring news, sent by formal government messenger (an ambassador, or "apostle"), of a military victory, the ascension of a new king, or the cancellation of debts and taxes. All of these were absolutely great news in the Roman Empire, because a military victory meant peace and security, the orderly ascension of a new king meant the continued benefits of functioning government, and I think we all know how good it feels to have taxes lowered or debts forgiven.
Well, this is precisely the language the first Christians took up to talk about Christ's work in his ministry, crucifixion, and resurrection. Paul and the other early Christian missionaries were official ambassadors for the kingdom of God, which had been established through the greatest battle and most unexpected victory of all time: the death of sin, death, and cosmic evil by Christ in his crucifixion and resurrection. With that a new kingdom was born and a new king took the throne, bringing them the benefits of the new divine kingdom: eternal peace and security, the forgiveness of our debts wrought through sin, and a functioning administration for the continued harvesting of the fruits of the new king's great victory.
The Romans hailed the Caesars as their lord and savior, the son of a god, and proclaimed his good news. But we have a far better Lord and Savior, Jesus of Nazareth, the Christ, the Son of God, with a far better gospel.