Among the various kinds of literature that circulated, Greco-Roman biographies was fairly dominant.1 This is relevant because, even absent their resembling biographies so much, it would recommend for us to take the gospels as biography as a matter of default, provided they are not letters.
The Gospels fit as biographies, internally exemplifying the quintessential family characteristics.
This page analyzes 11 arguments:
This is relevant because they specifically super-resemble early empire biographies, and were written in the early empire. And of course, absent word of the author's own explicit testimony, super-resemblance and bearing of the relevant features is the most you can ask for in determining a work's genre.
Rather than inventing Jesus-biography, the authors of Matthew, Mark, Luke, and John—as editors/redactors—were honest in writing their Gospel reports.
This page analyzes 8 arguments:
This is relevant because biographies are histories, and being a history was in fact the defining characteristic biographies (along with its emphases).
The gospels strove to be witness-based, recording witness testimony as closely as possible.
A full page will analyze 6 arguments:
This is relevant because historiography in general labored to be witness-based. (Relatedly, the gospels prized witness testimony, as did histories.)