Gospel originally meant the Christian message ("the gospel"), but in the 2nd century it came to be used also for the books in which the message was set out.
The word gospel came from the Old English word "gōdspel", which literally means "good news", since it narrates Jesus Christ's life and teaching to invite anyone to believe that he was born to save the world from sin and make humans truly know God as a Father. It includes the Death and Resurrection of Jesus.