According to the book of Luke, there were more writings about the Lord Jesus at that time. Only 4 books were selected to be included in the Bible more than 300 years after the Lord died. So if this new gospel book exists, it is not a surprise. Thomas loved the world eventually, and he left the Lord's ministry.
According to the most accountable records, St. Thomas, one of the great 12 apostles of Jesus, left for India and established his ministry in India. His church is still standing still till today in Madras (Chennai), Tamil Nadu. Some of his work in India could be found on the Acts of Thomas. I don't know from what accounts you drew your story that he'd left the church. The historicity whether the Gospel of Thomas was written by Thomas the apostle was still debatable, and I personally don't think this can be anyway easily resolved. So are the other Gospels, be they later accepted into canonization of the Bible or not. Anyhow, scholars these days have been extremely interested in this gospel, as it seems to be presented in an oral-transmission on fragmented accounts of actual Jesus preachings. And it also represents a convincing proof that the Q-documents (the earliest common source) that inspired writings of the Gospels of Matthew and Luke exist.