It is habitually called Gnostic, largely because it was found in the Nag Hammadi library alongside texts that plainly are. On its own contents the label sits badly. There is no demiurge, no fall of Sophia, no hierarchy of aeons, no cosmogony in which the material world is the botched work of an inferior maker, and no myth of any kind. What it has is a strong emphasis on hidden meaning, self-knowledge and the light within, which is compatible with second-century Gnostic systems but is not peculiar to them.
The wider problem is that Gnosticism has itself come under sustained attack as a category. Michael Williams argued in 1996 that it lumps together movements with little in common, and Karen King argued in 2003 that it was largely constructed by heresiologists and then inherited uncritically by modern scholarship. Whatever one makes of those arguments, applying the term to this text tells a reader less than describing what the text actually says.
Its relationship to Q, the hypothetical sayings source behind material shared by Matthew and Luke, is often stated too strongly. There is no literary dependence in either direction, and Thomas is not Q. The significance is generic: before 1945 the reconstruction of Q required believing in a kind of book of which no example survived, a collection of Jesus sayings with no passion narrative. Thomas is such a book, which made the hypothesis materially plausible. Koester pressed the further point that Thomas lacks the apocalyptic Son of Man sayings that Q contains, and drew conclusions about early Christian diversity that not everyone follows.
No church has ever treated it as scripture, and it appears in no canon, East or West. Its modern standing is scholarly and popular rather than liturgical: the Jesus Seminar printed it as a fifth gospel and voted on its sayings alongside the canonical four, and it is read widely outside any church by people who find in it a Jesus who teaches rather than dies. Both the scholarly use and the popular one should be described for what they are, and neither should be mistaken for a community holding it authoritative.