The [New Testament] Gospels are biographical narratives whereas most of the Gnostic Gospels are post-resurrection revelations. Typically in Gnostic Gospels Jesus appears to the disciples after the resurrection and, either in a discussion with a group of disciples or in a special revelation to one especially favored disciple, imparts knowledge of the true nature of the world and salvation, a message that is characteristically depicted as an esoteric revelation not given in Jesus’ public teaching during his ministry but reserved for the elect few to whom he entrusts it afterwards. That form of the Gospel, the post-resurrection narrative, actually presupposes that there are well-known accounts of the life and teaching of Jesus before his resurrection. Readers of these texts are certainly expected already to have some idea who Jesus and his disciples are. Some of these Gospels do not even name Jesus, but speak of him simply as the Savior or the Lord. But, more than that, the fact that they position themselves after the resurrection itself presupposes that definite accounts of Jesus’ teaching during his ministry are well-known. The purpose of the Gnostic Gospels is to add.
The very literary structure and nature of these “Gospels” presupposes well-known and widely-accepted preexisting narrative accounts of the life, death, and resurrection of Jesus.