The "Gospel of Thomas" does exist; however, not in the sense you're thinking. During the early eras many counterfeit epistles were written in the name of an apostle, or perhaps even an early church father. The purpose for this was mainly for either political gain, or theological motive. How do you think some of the earliest heresies such as gnosticism gained so much prevalence? It did so ultimately by fictitious autographs. What better way to spread a lie than to author a fictitious work and claim it had been authored by an apostle, and/or someone closely related to an apostle, such as a student?
And so, we have the "Gospel of Thomas," a writing completely out of tune with other NT witnesses. I think it is altogether fascinating that in all the works of the early church fathers, not one cites the Gospel of Thomas, or regard it as authoritative.