Bart D. Ehrman, The New Testament: A Historical Introduction to the Early Christian Writings, sixth edition (OUP, 2016), 97.
What kind of literature is a Gospel? Or, to put it somewhat differently, when ancient persons read or heard one of these books, what kinds of expectations did they have? Until recently, modern scholars generally agreed that the New Testament Gospels were unlike anything else in all of literature, that they were an entirely new genre invented by the Christians and represented by only four surviving works. The Gospels were obviously about the man Jesus and thus were somewhat like biographies, but compared to modern biographies they appeared altogether anomalous.
In one respect, this older view seems reasonable; as we will see in some detail momentarily, the Gospels do indeed differ from modern biographies. Scholars have nonetheless come to reject the idea that they are totally unlike anything else. There is probably no such thing as a kind of literature that is absolutely unique; if there were, no one would have any idea how to read it or know what to make of it. If people in antiquity could read the Gospels and make sense of them, then we have to assume that these books were not in fact completely foreign to them.
The question of how people in antiquity would understand a book should itself give us pause. While it may be true that the Gospels differ from modern genres like biography, they may not have differed from ancient genres. In fact, scholars of ancient literature have found significant parallels between the Gospels and several ancient genres. Some of these investigations have plausibly suggested that the Gospels are best seen as a kind of Greco-Roman (as opposed to modern) biography.