I recently read some very well-known gnostic writings that are under the name of "Gospel of Judah Iscariot" and "Gospel of Thomas", but they aren't written by Judah or Thomas. I don't really know what Gnosticism is. I read something about God having more "emanations", some "aeons", a "bad Demiurge" that created us. This doesn't make sense. I searched on Google about this. I understand that Gnosticism is a religion that add on top of Christianity 4000 years old Greek philosophy. I didn't really understands. What did those people that wrote those 2 gospels in the name of Judah and Thomas believe about Jesus. And the so called "Gospel of Judah Iscariot" seemed to me like a big heresy, but "Gospel of Thomas" didn't seem to be heretic or gnostic, in the text of it is presented a bit of the apostle's life like it is presented in the New Testament, there are presented some of Jesus's statements about loving enemies, helping poor. I didn't see anything about God "emanations", "aeons" and a "bad Demiurge", the "Gospel of Judah Iscariot" was full of those aberrations, so I don't understand why scholars call the "Gospel of Thomas" as being gnostic.
Early Christianity specialist professor of Religious Studies Nicola Denzey Lewis wrote a short web article that answers your question: Was the Gospel of Thomas Gnostic? As a PhD student, Ms. Lewis studied under Dr. Elaine Pagels, a professor of religion who wrote the 1979 groundbreaking and controversial study: The Gnostic Gospels.
Before addressing your questions, let's first explore why we can be sure that the early church considered that only the 4 gospels we have are canonical. A Church grammar podcast episode John Meade on the Development of the Biblical Canon, Canon Lists, and Origen the Text Critic touched briefly on the Gospel of Thomas (min 28:30) saying that although people like Origen may made use of a portion of it
- none of the church fathers ever said that the Gospel of Thomas was inspired and
- the subcollection of the 4 gospels were established as early as St. Irenaeus
Your first question: why the Gospel of Thomas is considered Gnostic?
First we need to realize that it contains 114 sayings that are NOT placed in a narrative framework like the Gospel of Mark, making it hard to interpret, and which enables scholars to have 2 theories, whether it was the source of Mark or whether it is an abbreviated version of Mark. Recent research has argued for the latter. Regardless of the dating, because it doesn't have narrative framework, some sayings are more consistent with Mark's portrayal of Jesus, but some are NOT. If we start with trusting the canonical portrayal of Jesus as authentic, then we can say that the sayings that don't are later additions more consistent with Gnostic teachings.
Your second question: How is the teaching of Jesus different than the 4 canonical gospels?
Nicola Denzey Lewis made the following 2 points in her article (emphasis mine) which I think made it clear how different the teaching is from orthodox Christianity:
According to the scholars who originally worked on the Gospel of Thomas, the text contained two Gnostic elements. First, it purported to contain “hidden” or “secret” knowledge (Greek, gnosis) that, if properly understood, could lead a keen reader to salvation. In the classic formulation of German scholar Hans Jonas’ The Gnostic Religion (1960), gnosis is the salvific knowledge of one’s spiritual origins. The Gospel of Thomas states early on that gnosis is its chief goal: “Whoever discovers the meaning of these sayings will not taste death” (GosThom 1). Second, Jesus behaves differently in the Gospel of Thomas than he does in the canonical gospels. He gives very strange advice, and there’s no account of his crucifixion and resurrection. Even more remarkably, the Gospel of Thomas’ Jesus seems to suggest that a reader who gains proper gnosis can actually become not a Christian but a Christ. Rather than placing their faith in Jesus, readers are encouraged to seek their inner Christ to find salvation.
For further details, I think Dr. Nicola Denzey Lewis's 2012 textbook Introduction to Gnosticism: Ancient Voices, Christian Worlds is a good resource.