I didn't say the Gospels were fiction. I asked about a source that they were written by eyewitnesses. I'm a l;azy researcher, but I recall my readings that they were written at the end of the first century and in the second. I welcome evidence to the contrary.
Luke was St. Paul's secretary. Not an eyewitness, but took down the testimony of many eyewitnesses in his capacity as scribe to a very active Apostle. But, as I said earlier, much eyewitness testimony is taken down by another rather than written by the eyewitness himself (e.g. Tacitus and Pliny the Younger).
Here is a site that goes into excruciating detail regarding the historicity and authorship of the Gospels. From my observation, the author has a good working knowledge of the classical texts as well as the Church Fathers.
They have different authors and at least one of them wrote without reference to the others. It seems as though they were setting down in writing what had been common report but with a different purpose in mind. We can also add the testimony of Paul, whose occasional letters mention a meeting with men who knew Jesus personally. It may be reasonable to say that they were "fictionalizing" (based on a true story) and saying that Jesus was a totally ficticious character.
The Gospels believed to be written by Apostles are Matthew and JOhn. Mark and Luke came later.