After delivering the book of Revelation, members from all around the Church came to see John and get his story from his time with Jesus. The Gospel of John was written in 97 AD and that's 60 years after Jesus, therefore John was just making stuff up.
What people fail to understand is, obviously John didn't just suddenly have to remember what it was like to be with Jesus decades ago. Those three years with Jesus he would have been thinking and telling others about his whole life. The Gospel of John was just routine to him by that point.