The Gospel of Thomas is one of 300 Gnostic writings discovered in December 1945, at Nag Hammadi, Egypt. This writing exists completely only in the Coptic language; there are three fragments of it in Greek.
According to Anthony Saldarini in the New Jerome Biblical Commentary, of the 114 sayings of this gospel (division made by modern scholars), 79 of them have some parallel in the Gospels of Matthew, Mark and Luke which, with the Gospel of John, are called canonical. They belong to the canon or list of New Testament writings.
The Gospel of Thomas is called apocryphal because it is not in the New Testament. The Gospel of Mary Magdalene is also in this category.
Gnostics were people who relied on secret knowledge; gnosis is the Greek word for "knowledge." Special teachers enabled Gnostics to hand on secret information not intended for everyone. In that general sense, Gnostics could be pagan, Jewish, or Christian.
Christian Gnostics claimed that Jesus wanted only a few of his followers to have the teachings they possessed. This knowledge was handed on by Gnostic teachers.
The mainstream Christian community answered the Gnostic challenge by saying that Jesus intended bishops, successors of the apostles, as reliable teachers about him. They also said that the Christian community considered as inspired only the canonical books in the New Testament and the Old Testament.
The Scriptures were given to a faith community (Old Testament to the Jews, New Testament to Christians). They should be read with that in mind. If we trust those faith communities enough to tell us which writings are inspired by God, should we not also trust them to interpret them?
These Gnostic gospel texts, as well as more background, are probably available at your public library.