The Gospel simply put is "The Good News" of what Christ has done for you and me. This is the message and meaning of who and what Jesus is and has done.
It means from the Greek, "A Good Message" literally or more to the point "Good News" referring to a message of good tidings such as...
This was the rallying cry of the Reformation. The main point that caused Luther and Calvin to challenge the Catholic Church and it's teachings.
The Gospel is the lightning rod that conducts God's creative and loving power, because it reveals the justice of God in the justification of the ungodly!