The Gospel of Thomas is a collection of sayings attributed to Jesus of Nazareth. Unless it is merely a collection of materials that mainly were drawn out of the Biblical gospels, as seems unlikely for most if not all of Thomas's ayings, then Thomas is the most important historical source for knowledge of Jesus of Nazareth that exists outside of the Bible. It is the most significant manuscript ever found for the history of earliest Christianity. When the gospel was written is question many scholars are debated. Scholars say that it was written at about the same time, even perhaps somewhat before, the gospels in the bible. Their argument is that most of the sayings in Thomas show no signs of having any dependence on, or knowledge of, the Biblical gospels and so Thomas's ayings derive from oral tradition and not from written Biblical texts.
This doesn't seem to have been possible after the end of the first century when the Biblical texts began to be authoritative in Christianity. Other scholars find bits of evidence that indicate that Thomas was indeed dependent, in part, on Biblical texts, and surmise that the author of Thomas must have edited out almost all indications of the particular styles and ideas of the Biblical authors. Those scholars date Thomas in the mid second century A. D. Another great question is of who wrote the gospel. No one knows.
The four canonical gospels and Thomas and other gospels such as the Gospel of Philip (found at Nag Hammadi) were given their names some time in the second century. Scholars of the New Testament generally agree that none of the gospels were written by people who had ever met Jesus of Nazareth during his lifetime. But at a later date names were assigned to them that were associated with famous individuals in the earliest church. The name of the person who supposedly wrote the Gospel of Thomas is given in the first lines of the text as "didymos Judas Thomas." The word "didymos" is Greek for twin and the word "Thomas" is Aramaic for twin. The individual's name was Judas, and his nickname "the twin" is given in two languages. The canonical gospels mention a man named Thomas and John calls him didymos Thomas.
There are also several individuals named Judas mentioned in the canonical gospels in addition to Judas called Iscariot. The bottom line is that we do not know who wrote the Gospel of Thomas and we cannot be sure which Judas mentioned in the New Testament also was nicknamed Thomas. Portions of three Greek copies of the Gospel of Thomas were found in Oxyrhynchus Egypt about one hundred years ago. They are known as Oxyrhynchus Papyrus 1 (Oxy P 1) probably written not much later than the year 200, Oxy P 654, which can be dated to the middle or end of the third century, and Oxy P 655 dated not later than A.
D. 250 (dating according to Grenfell and Hunt). A complete version in Coptic (the native Egyptian language written in an alphabet derived from the Greek alphabet) was found in Nag Hammadi Egypt in 1945. That version can be dated to about 340 A.
D. The Coptic version is a translation of the Greek version. Most scholars believe that the Gospel of Thomas was originally written in Syria in the Greek language. The basic perspective is that the Kingdom of God is spread out upon the earth now, if people can just come to see it; and that there is divine light within all people, a light that can enable them to see the Kingdom of God upon the earth. Further, the perspective of Thomas is that the Image of God in the beginning (Genesis chapter One) still exists and people can assume that identity, an identity that is neither male nor female. The image of God is differentiated from the fallen Adam of Genesis chapter Two.
The Gospel of Thomas advocates that people should restore their identities as the image of God now, and see the Kingdom of God on earth now. Thomas reads the first two chapters of Genesis in a straightforward way, there were two separate creations of mankind; the first is perfect, the second flawed. Rather than waiting for a future end-time Kingdom to come, Thomas urges people to return to the perfect Kingdom conditions of Genesis chapter one. For Thomas End zeit (the final culmination of things) already existed in the Ur zeit (the primordial creative time of the past). How many words come from Jesus in the gospel Who knows for sure If you take the set of sayings that are in Thomas and that are also in the gospels of Mark or Matthew or Luke (no sayings in Thomas are also in John) then you have a set of sayings that rather reliably come from Jesus.
Scholars commonly are so influenced by biblical texts that they assume that any sayings in Thomas that don't sound like sayings in Matthew/Mark/Luke are therefore not sayings of Jesus. However, it is quite possible that Thomas retains sayings that the biblical gospels don't retain and, indeed, that Thomas is more reliable as a guide to the sort of thing Jesus said than the biblical gospels are. Matthew/Mark/Luke give by and large the same point of view regarding Jesus as a teacher. Thomas (and to some extent John) gives a somewhat different point of view. Perhaps Thomas' point of view derives from Jesus himself. Or, perhaps, not.
The Gospel of Thomas is, as you may know, not in the Bible. And probably never will be. The biblical canon is not open for debate, it is a closed entity. A church that adds Thomas to its collection of scriptures would move outside the margins of orthodox Christianity and no well-known denomination has the slightest intention of adding Thomas to its scriptures.