The belief that "The Gospel of Thomas" should be considered as authoritative and divinely inspired as the traditional gospels is based upon the assumption that the Thomas "gospel" was written either at the same time as, or perhaps even earlier than, Matthew, Mark, Luke and John.
However, if we consider the possibility that this text was originally written in Syriac and likely taken from its source, Tatian's Diatessaron, we no longer have the "first-hand" sayings of Jesus, do we?
For those interested in investigating and verifying...
THOMAS: THE FIFTH GOSPEL?