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.
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.
"Injil" (Arabic: إنجيل) is the Arabic name for the Gospel of Jesus (Isa). In Islam, Jesus is believed to be the second-to-last prophet and messenger of God (Allah) and the Messiah. Muslims believe that God revealed the Injil to Jesus, while also declaring the truth of the previous revelations: the Tawrat (the Torah) and the Zabur (the Psalms).
Media related to Gospels at Wikimedia Commons