
No links, just prolific reading from books and magazines over the years. Keep in mind, the same is true for any Bible book. The dates they were written do not match the date of the oldest copy in existence. Scholars have numerous ways to estimate when an account was actually written. For example, if a book has a prophecy that the Temple would fall, it is fairly certain the account was written after the Temple did fall.
As far as different dates for the oldest existing copy: Shrug. I don't know. Perhaps better dating? Perhaps a disagreement among scholars? In any event, even though it is postulation that John's Gospel argues Thomas' Gospel, we do know Origen mentioned the Thomas Gospel was a pack of lies--and Origen lived in the third century. I doubt that everything in the Thomas Gospel is a lie or in error. I imagine the Gnostic foundation was even more apparent back then than it is now. From the first, Gnostics and Christians appear to have disagreed about what Jesus meant.