Quote:If the gospels were in circulation for many years before they were ascribed to the authors whose names they now bear,
That's a big "IF" you have there at the beginning of your question. Seeing as how Justyn Martyr writing c 160 never heard of any of them I would say that the condition you laid out is doubtful, at best.
Early xtian writers tell us that it was Marcion of Sinope who produced the first xtian canon. "The Gospel of the Lord" and 10 epistles of the so-called "paul." The gospel of the lord turns out to be what we now call "luke" after a couple of chapters were added to it.
The observation that Marcion was the first to write things down in some sort of authoritative book opens up a realm of possibilities, not the least of which is that jesusism or whatever you want to call it at that time, was a mystery cult like all the other popular mystery cults in which knowledge was conveyed orally from masters to initiates. Marcion was the innovator and, to give them credit, the proto-orthodox writers of the late 2d century saw the utility in writing this shit down. But if they did so, they were going to make sure that it said what they wanted it to say.