"So what's the date of the earliest surviving manuscript of the canonical Gospels?"
Most believe he oldest "new testament" writings are those by Peter (his letters), which are obvious near-contemporaneous writings, given Peter's well-documented (by Christian and non-Christian sources) execution by Rome not too long after Jesus.
Peter's letters clearly state, or require as knowledge to make sense, many or most of the accounts found in the gospel --- most notably the death and resurrection of Jesus.
In sum, my somewhat non-answer to your question is the gospels-written-long-after dispute is a bit of a red herring.
The point is that if the date of the 'Judas' manuscript is being raised, as it was, one should also point out that except for a few fragments, the canonical manuscripts date no earlier than the fourth century. And while Judas is younger than most of the canonical NT, it may well be within the range of the later books.
I do agree news coverage has been superficial and somewhat tendentious, and that this m/s is nothing new to anyone familiar with the gnostic gospels and other works.