Starting with the early church, a large portion of Christian tradition has held the Augustinian hypothesis which believes that Matthew's Gospel was the first to be written, then Mark's, then Luke's, and subsequently John's, with each writer having some knowledge of the others' writing.
Since the 19th century however, many biblical scholars have rejected Augustinian hypothesis (namely that Matthews Gospel was the first to be written) in favor of Markan priority. On top of this, they assert that Matthew and Luke's Gospels have used Mark's as a source from which both draw virtually identical narrations (see The Synoptic Gospels). Under this hypothesis, those portions of Matthew and Luke that are in agreement but do not appear in Mark are believed to have been borrowed from an alternate source commonly referred to as "Q"
"Q" as a tangible fragment or codex has never been discovered within the field of archaeology and paleography however. Many biblical scholars defend "Q's" lack of tangible existence due to many portions of scripture being lost or edited.
However, one group certainly must have recorded an alleged Gospel as important as "Q" would have been, and that group is the Early Church Fathers.
So, do any Early Church Fathers record of another Gospel similar in content to those portions of Matthew and Luke that do not appear in Mark?