The "Gospel of Thomas" has been presented in the media as the forgotten gospel which is needed to set straight the message of the New Testament. It is not a bible hoax in the usual sense of the word, but an actual ancient manuscript. However, it is not a real gospel.
The Church Fathers Hippolytos and Origen knew about a Thomas gospel, the contents of which they did not recognize as genuinely Christian. But the actual book has only existed in fragments since the end of antiquity. In the late 1940s, a complete version was found at Nag Hammadi in southern Egypt. It was part of a library of more than 40 gnostic scriptures. The manuscripts were from the 4th century, copies of older originals.
Since these scriptures were published and translated, they have been examined by exegetic scholars around the world. The Thomas manuscript is a collection of 114 Jesus utterances. About half of them are similar to utterances in the canonic gospels, but all are slightly different. To some extent, contents and meaning have been altered. The biggest difference to the NT is that there is no narrative material about the life of Jesus. His utterances are given without any reference to when and where they were spoken. Similarly, there are no discussions with Jewish learned men, and all references to the Old Testament are missing.
There is no trace of the central NT story - Jesus' birth, death on the cross, and resurrection. When the Thomas gospel speaks of the Divine life, it is without reference to the empty tomb of Christ. The document has no interest in the historic Jesus, only the "living Jesus". This transforms him from the saviour of mankind to just a teacher. No admonitions to repentance or conversion are present; everything is about knowledge and spiritual insights.
Scholars immediately recognized that the Thomas gospel is a gnostic book. Several other manuscripts from the same find even more clearly belong to this philosophy. The differences to the Biblical gospels are so vast that we should not be surprised that this book was not included in the Bible, regardless of its age. There is no evidence for that it was used in regular Church services either.
Gnosticism has some ideas in common with Christianity. It uses a similar language e.g. in speaking of light and darkness, like the gospel of John. But in its basic approach, gnosticism is completely alien to Christianity. It does not offer faith in a redeemer, only insights into the secrets of life. It regards the material world as completely separate from the spiritual reality, in a way which excludes the Biblical belief in creation. This leads to a morality which either becomes extremely ascetic or completely libertine/hedonist. In the present age when Christianity is being diluted or weakened in the Western world, the gnostic ideas are again on the agenda. The gospel of Thomas fits right in: it presents Jesus as a teacher of wisdom, separated from the time and place where he lived.
At the core is the question of whether Man needs a saviour, or whether we are able to save ourselves by reaching a sufficient spiritual "level". The former is Christianity, the latter is Gnosticism.