Thomas is the apostle referenced in the Gospel of John. He is described as the apostle of wavering faith refusing to accept that Christ was risen until he had touched the wounds he received on the cross.
Athanius The Apostolic was the 20th Pope of Alexandria and in his Easter letter of AD367 he was the first to list the 27 books of the New Testament canon. The Gospel Of Thomas was not included.
There is a wide variance of opinion regarding this. There are academics who have made the case that it was written before the Gospel Of Mark AD70 whilst others claim it was not written until the latter half of the second century.
The Apostle Thomas is the disciple who is purported to have both written the gospel and also to have a special understanding of the teaching it conveys.
