Valantasis’ The Gospel of Thomas (1997) to be a good starting point, and he makes very clear a number of problems in dating the text:

• What do we mean by “text”? As Thomas is a collection of sayings (as opposed to a linear narrative or singular viewpoint), and has various styles within the text, it’s highly likely that it was composed by multiple authors at different times. Some may be earlier, some may be later, but the definitive text was probably mid-late 2nd Century and obviously no later than 250 CE.

• DeConick argues in Recovering the Original Gospel of Thomas: A History of the Gospel and its Growth (2006) that some (DeConick names five) of the sayings may be a “kernel”, written as early as 30-50 CE (predating all the canonical Gospels) and used by the Jerusalem mission.

• You can use various methods to deduce the date, but it takes very subtle and nuanced analysis that is open to interpretation. Valantasis looks at comparisons to the other Gospels, source analysis, theological comparison to (later) “Gnostic” philosophy, wisdom literature, Pauline comparisons, historical context of the early Christian “church”, etc. all of which can come up with different dating assumptions (e.g. that “collections of sayings” are more similar to 1st century than later narrative-driven Gospels)

• Valantasis places it roughly 100-110 CE, aligned (for him) with John and the Letters of Ignatius, but that some material is likely earlier (30-60 CE).

Meyer in Jesus Then & Now: Images of Jesus in History and Christology (2001) says that a 1st century dating of Thomas is “reasonable though probably not compelling”, and also makes the case that Thomas was an additive collection (and actually says we should think of it more as the Gospels of Thomas in that way).
DeConick argues that there was a Kernel Thomas originating “from the mission of the Jerusalem Church between the years 30-50 CE.” http://aprildeconick.com/gospel-of-thomas
“The earliest possible date would be in the middle of the 1st century, when sayings collections such as the Synoptic Sayings Gospel Q first began to be compiled. The latest possible date would be toward the end of the 2nd century, prior to the copying of P. Oxy. 1 and the first reference to the text by Hippolytus. If Gos. Thom. is a sayings collection based on an autonomous tradition, and not a gospel harmony conflated from the NT, then a date of composition in, say, the last decades of the 1st century would be more likely than a mid-to-late-2d-century date.” (The Other Gospels: Non-Canonical Gospel Texts, pg 536)

1. According to Deconick:
2. It began as a smaller gospel of Jesus’ sayings, organized as a speech handbook to aid the memory of preachers. I call the earliest version of the Gospel of Thomas, the Kernel Thomas. The Kernel Thomas originated from the mission of the Jerusalem Church between the years 30-50 CE. It was taken to Edessa where it was used by the Syrian Christians as a storage site for words of Jesus. Its main use in the Syrian Church was instructional. The Kernel sayings were subjected to oral reperformances, which was the main way that the text was enhanced with additional sayings and interpretations. This does not mean that literary sources did not effect its growth, but that the process was not one of an author sitting down with a pen in hand and editing a couple of written sources together.
3. John Dominic Crossan dates the first layer of Thomas to about 50.