One when picks up Robinson's Redating the New Testament in 2017, one cannot but be struck by a lacuna in the texts that he treats. He considers the entirety of the New Testament, and in a "Post-Apostolic Post-Script" also treats certain of the Apostolic Fathers (notably, 1 Clement, Didache, Epistle of Barnabas, Shepherd of Hermas). He does not however treat the Gospel of Thomas, which in light of its position in subsequent scholarship is a notable absence. No doubt, it's because in 1976 the arguments that would place Thomas, at least in its earliest forms, in the fourth or even third quarter of the first century had not yet taken on the prominence they later would. Given the existence of arguments from quite competent and respected scholars that would situate the Gospel of Thomas as coeval or just slightly later than the last of the New Testament texts, it would at the very least be altogether uncollegial to not address the issue of Thomas' origin in a study on the dates of the New Testament written today.
An initial consideration is the matter of "gnosticism." A first observation on the matter: just as recent scholarship has recently argued that the "high" Christology of John's Gospel cannot be correlated directly with a later date, neither could any "gnosticism" detectable in Thomas'. A second observation: the tendency to treat Thomas as a gnostic text has receded in recent years. It's now increasingly recognized that it was slotted into that category in large part due to guilt by association. The complete Coptic text being found alongside the trove of "gnostic" texts found at Nag Hammadi, people supposed that it must fall into the same category. Yes, just as we cannot suppose without argument that the canon espouses a consistent theology, neither can we suppose the same for the Nag Hammadi library. A third observation: it's not clear to me that the wisdom- or gnostic- oriented Christianity on display in Thomas' Gospel is significantly more "advanced" than that with which Paul is engaged in the Corinthian correspondence. Indeed, there is sufficient resonance that it has been argued that his Corinthian opponents included persons influenced by Thomasine thought. Whether that is the case or not with regard to Paul, the salient point is that the conditions necessary for the emergence of something like Thomas' Gospel probably existed by c. 60.
With that in mind, I am struck by two logia in the Gospel of Thomas. One is logion 12, in which Jesus is asked by his disciples who will lead them after he is gone. He tells them that they should go to James the Just. Now, it seems difficult for me to locate the origin of this logion much after the death of James in 62. The later one pushes it past that date, the less useful an answer it is for those who would want guidance regarding post-dominical leadership. What use is it to someone in 150 to hear that they should go to a person dead for almost a century? It's also not clear to what extent after the 60s there were Christian leaders that based their legitimacy upon any sort of Jacobean authority, thus obviating the (really quite speculative) argument that it refers to those who claim that authority rather than to James himself. Certainly, the most natural place for this logion is before 62.
The second logion of interest is the very next one, 13. In this logion, Jesus asks his disciples to compare him to someone. Peter says that he is like a righteous angel. Matthew says that he is like a wise philosopher. Thomas says that he cannot articulate a comparison. What interests me is that Thomas' Gospel singles out specifically Peter and Matthew here. Now, these just happen to be the members of the Twelve associated with those canonical gospels generally held to be the earliest written: Mark's and Matthew's. Moreover, interestingly, they are listed in the order that most scholars suppose they were written. Is this just coincidence? Those who would state that Thomas' Gospel is independent of the canonical gospels would probably have to conclude "Yes," but one wonders whether that is the best judgment or not. Those who see the Gospel of Thomas as dependent upon one or more of the canonical gospels could answer "No, it is not a coincidence at all."
Given the above statements, it must be noted that there are a series of affirmations that only one who dates the Synoptic Gospels early (i.e. before c. 60) can most readily affirm simultaneously. One who dates the Synoptic Gospel thus early one could affirm that Paul is engaging with an articulation of Christianity similar to that found in Thomas' Gospel; could affirm that the most natural reading of logion 12 situates it as a pre-62 reference to James the Just's leadership; could affirm that there is a compelling logic to the selection of specifically Peter and Matthew from among the Twelve in logion 13; and could affirm the recent (and in my mind convincing) arguments that Thomas either knew all three of the Synoptic Gospels or emerged from Christian circles closely related to those which produced the Synoptic Gospels. Moreover, one could do so without positing hypothetical proto-Thomases that pre-date the "final" form. A lower chronology permits a strong synthesis between competing schools of Thomasine studies that are excluded by the middle and higher chronology. The strength in such synthesis does not lie simply in presenting a diplomatic or compromise position, but rather in the possibility that rather than two diametrically opposed schools, one of which is right and one of which is wrong, what we have is two groups of scholars who, coming at the same material with differing subjectivities, are each apprehending vital aspects of the same reality about the Gospel of Thomas. When their respective positions are brought into dialogue, and chronological barriers removed, perhaps the fuller reality becomes apparent.
Now, let me be clear: I'm not arguing that Thomas' Gospel should be dated c. 60. The above is a hypothesis, one which at this point I neither affirm nor reject. I have not yet thought through the issue sufficiently to reach a final judgment. But one who would undertake to argue that many of the texts of the New Testament canon are notably earlier than typically supposed cannot in principle exclude the possibility that the same is the case for some of the New Testament apocrypha. It is thus incumbent upon me to explore such possibilities, considering and vetting hypotheses for such earlier dates. Any other procedure would run the risk of special pleading.