According to Mark’s Gospel, Jesus begins his public work with a proclamation. After his baptism by John and his temptation in the wilderness, Jesus “came into Galilee, preaching the gospel of God, and saying, ‘The time is fulfilled, and the kingdom of God is near: repent, and believe the gospel'” (Mark 1:14-15, my translation). Matthew and Luke present basically the same message.
Let’s look closely at what Mark’s narrator and Jesus say here. The Gospel describes Jesus’ message as the gospel. Jesus challenges people to “repent, and believe the gospel.” In between, Jesus proclaims “The time is fulfilled, and the kingdom of God is near.” That core announcement — “It’s time, and God is breaking into the world” — that is the core of Jesus’ own gospel.