The skeptics who make this argument usually claim that the Gospels were written anonimously and later credited to their supposed 'authors' by mere guesswork, probably in the middle of the second century A.D. This argument defies all common sense, not to mention historical evidence.
Historians consider the authorship of any ancient document verified if all existing copies credit the same author. Unless there is some sort of disagreement, a lack of authorship credit on early documents, or some other logical reason to doubt that the supposed author is falsely credited, they don't give much doubt to the validity of the credited author.
It is rare to the extreme for any ancient document to be untitled. People needed some way to differentiate scrolls or codex texts from others. The idea that any author would write a text and not put a title on it makes about as much sense as a writer from today leaving a writing untitled.
Yet skeptics claim the Gospels were originally untitled. Is this because we have early copies which are untitled? No, even the earliest copies of the Gospels are clearly labeled with the names of Matthew, Mark, Luke and John. Is it because such widely read texts would logically be left untitled? No, the larger the intended audience the more likely a text is to have a title in order to avoid confusion.
So the skeptics are merely guessing that there once existed copies of the Gospels which had no titles on them. Such guesswork without evidence, which flies in the face of reason, is laughable.
Early historian Irenaeus (circa 175 AD) confirms the authorship of the all four Gospels, as does Diatessaron (circa 170 AD). Early historians Papias (circa 125 ad) and Origen (circa 240 ad) all confirm that Matthew was the writer of the Gospel attributed to him. Mark's authorship is also confirmed by them, as well as Justyn Martyr (circa 150 ad), Clement of Alexandria (circa 200 ad), and Tertullian (circa 207 ad), among many others. Luke's was also confirmed by Clement of Alexandria, as well as by Muratori and Tertullian. Theophilus (circa 170 ad) confirms the authorship of John.