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:
So, as far as their internal features, the Gospels specifically super-resemble early empire biographies, and we know they were written in the early empire era. This is not to say these features are all unique to Greco-Roman biographies, but rather that together they show the Gospels have a "family resemblance" to the genre which is quite decisive. And so of course, absent word of the author's own explicit testimony, this resemblance and bearing of the relevant features is the most you can ask for in determining a work's genre. This all successfully suggests the Gospels were indeed patterned after Greco-Roman biographies and ask to be read as such.
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.)