I’m working on Gospels and tackling the genre question in more recent days. Here is my interim definition (with obvious HT to Talbert, Burridge, and Aune):
The Gospels are the textual imprint of the oral phenomena of Christian preaching and teaching about Jesus. Viewed this way, the Gospels are Christian documents related to the needs of Christians in corporate reading, worship, apologetics, and proclamation. So in that sense they are a unique genre with no precise literary counter-parts. However, their uniqueness is in many ways inconsequential because they remain generally analogous to Graeco-Roman biography and the biographical genre was typified by innovation and adaptation. The content of the Gospels is singularly determined by Jewish Christian content, while the literary form of the Gospels is a clear sub-type of Graeco-Roman biography.
It would seem that Mark has, consciously or unconsciously, adopted Graeco-Roman biography as the literary vehicle to produce the first Jesus book, narrating Jesus’ career, his teaching, and his death. It is a literary work that was composed in light of Mark’s apocalyptic worldview and combined with an intimate intertextual dialogue with Israel’s sacred traditions. So, as Hurtado recognized, composing a biography may not have been the specific aim of Mark, and the resultant literary form was perhaps more indebted to the general climate than to anything else that contributed to his decision to make his pioneering literary work a biographical-like narrative.Yet in any case, the result was a profound adaptation of the biographical genre through the convergence of Christian storytelling about Jesus along with close connections to Jewish sacred writings, all set in the coordinates of a continuous narrative account of Jesus’ career and death. Thus the Gospels can be likened to Graeco-Roman biography, but are also form a distinctive family within that body of ancient writings. For this reason, I label them “biographical kerygma.”
 Aune, New Testament in Its Literary Environment, 46. Note also that Guelich (“The Gospel Genre,” 174, 205) questions the sui generis designation for the Gospels since genre is premised on a “context of expectation,” implying that no genre can be so unique as to be inexplicable and its intelligibility is based on prior and analogous reading experiences; and all genres are by definition unique, so the statement “unique literary genre” is redundant.
 Hurtado, “Genre,” 280-82.