Just putting some thoughts on paper here. The first recorded genre classification of the Gospels were by Justin Martyr. He writes,
All who live in cities or in the country gather together to one place, and the memoirs of the apostles or the writings of the prophets are read, as long as time permits (Apology I 67.3)
So, some have taken this to point to the ancient biographies although few knew the difference between ancient and modern biographies. But, I was thinking… Maybe Justin, the Platonist, used the word in a similar fashion to Xenophon who wrote some memoirs of Socrates (So-crates). These Memorabilia of Socrates presents the teacher as defending himself against the charges of the Athenians, but unlike Socrates’ Apology by the same author, this work is a running commentary on sayings and other things of Socrates meant to prove the innocence of the philosopher who died for his people. It is filled with dialogue, narrative, and other genres cobbled together by Xenophon, not as a matter of recording history but as a matter of preserving truth. Tons of stuff in there.
I’m not saying Mark, Matthew, Luke, and John were writing to in the style of Xenophon; however, their style may have allowed Justin to place the two together, especially if the early Church understand the Gospels as a defense of Jesus rather than a record of Jesus.