The word gospel 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 is mostly about the Death and Resurrection of Jesus.
The Gospel was originally spoken, not written.
Eventually gospel came to mean an ancient book about Jesus, especially one of the four books collected by the Church in the Bible.
These books are the first part of the New Testament of the Bible, and are put in a group of this order: the Gospel of Matthew, the Gospel of Mark, the Gospel of Luke and the Gospel of John.