What is the Gospel of Thomas?
The GTh is "an anthology of 114 'obscure sayings' of Jesus, which according to its prologue, were collected and transmitted by St. Didymus Jude Thomas. The sayings do not appear within a biological narrative about Jesus, although some of them individually contain elements of dialogue or an abbreviated setting. Instead, Jesus' sayings in Gth are unconnected and in no particular order." It is part of a collection of Gnostic writings known as the Nag Hammadi Library.

The Nag Hammadi Library is a collection of Coptic documents, found in Upper Egypt in 1945, dated late fourth century AD:

The most important collection of Gnostic writings are the Nag Hammadi Codices (NHC). Thirteen codices, containing fifty-two tractates, were discovered in upper Egypt in 1945. Six of these tractates were duplicates. Six others were already extant. The remaining forty represented wholly new finds.

The manuscripts are dated to the late 4th century, on several grounds, the two strongest are

First, there is a reference in Codex VI (containing The Concept of our Great Power) to the heresy of the Anomoeans--which briefly flourished in the region in the late 350's.

Second, some of the 'packing materials' in the jar are literary pieces themselves (like we might use newspaper to pack a box of delicate objects). There are three dates that show up in these packing materials of Codex VII: 341, 346, 348 ad. "This indicates that the cover of Codex VII was manufactured no earlier than the latest date [348ad], but perhaps as much as a generation after these dates."

What does the GTh contain?

The Coptic Gospel of Thomas was translated from the Greek. Fragments of this gospel in the original Greek version are extant in the Oxyrhynchus Papyri 1, 654 and 655, which had been discovered and published at the beginning of this century, but were identified as parts of The Gospel of Thomas only after the discovery of the Nag Hammadi Library. The first of these Greek papyri contains sayings 26-30, 77, 31-33 (in this order!), the other two the sayings 1-7 and 36-40, respectively. At least one of these Greek fragments comes from a manuscript that was written before 200 C.E.; thus the Greek version of this gospel was used in Egypt as early as the second century.

A large number of the sayings of The Gospel of Thomas have parallels in the gospels of the New Testament, in the Synoptic Gospels (Matthew, Mark, and Luke), as well as the Gospel of John (parallels with the latter are especially striking: cf., e.g., sayings 13, 19, 24, 38, 49, 92). Some of the sayings are known to occur also in noncanonical gospels, especially in the Gospel According to the Hebrews (cf. saying 2) and the Gospel of the Egyptians (cf. saying 22), which are both attested for the second century by Clement of Alexandria (floruit 180-200).