Biographies are narratives but they are narratives without rhythmic structure (like poetry); instead they follow a natural flow of speech with ordinary grammatical structure. This is relevant because the Gospels too are continuous prose narrative.
But so what?
• Romances are continuous prose narrative as well. [Response forthcoming.]
Greco-Roman histories self-identified as relaying witness testimony.
Historical works from the Greco-Roman mediterranean proudly self-identity as relaying witness testimony.
• Polybius etc. testify that they did.
• Historians oft say: “I saw this all 1st hand.”
• Histories cited witnesses via emphasis.
• Histories DID get witness-testimony or close.
But so what? Plausibly...
• The Gospels did not claim to relay witness-testimony.
Unlike dramas (for example), biographies in this era tended to be between 10,000 and 25,000 words long. This is relevant because the Gospels are 11-19k words long. (Mark has 11,242 words; Matthew has 18,305; Luke has 19,428; John has 16,150).1
Unlike most other genres (which may focus on things like an time period, or a large-scale event, or the goings-on of a government, Greco-Roman biographes focused their attention throughout the work on a single character.
Unless other genres, Greco-Roman biographies placed a heavy emphasis on trying to morally capture the virtues and vices and other characterstics of their subject through their choice of which words and deeds to narrate.