The Gospel of Judas is a Gnostic gospel whose content consists of conversations between Jesus and Judas Iscariot. It is thought to have been composed in the second century by Gnostic Christians, not by Judas, since it contains late-2nd-century theology. The only copy of it known to exist is a Coptic language text that has been carbon dated to AD 280, plus or minus 60 years. It has been suggested that the text derives from an earlier Greek version. A translation of the text was first published in early 2006 by the National Geographic Society.
In contrast to the canonical gospels, which paint Judas as a betrayer who delivered Jesus to the authorities for crucifixion in exchange for money, the Gospel of Judas portrays Judas's actions as done in obedience to instructions given to him by Jesus of Nazareth. It does not claim that the other disciples knew about Jesus's true teachings. On the contrary, it asserts that they had not learned the true Gospel, which Jesus taught only to Judas Iscariot, the sole follower belonging to the "holy generation" among the disciples.
So we see that it was not only not written by Judas, it's teaching are contrary to what the synoptic gospels tell us about Jesus.
Overview
The Gospel of Judas consists of 16 chapters which document Jesus's teaching about spiritual matters and cosmology. According to the text, Judas is the only one of Jesus's disciples who accurately understands the words of his master. This Gospel contains few narrative elements; essentially, the Gospel records how Judas was taught by Jesus the true meaning of his message.
The Gospel contains ideas which contradicted those circulating in the early Christian church. The author says that God is essentially a "luminous cloud of light" who exists in an imperishable realm. Adamas, the spiritual father of all humanity, was created in God's image and dwelled in the imperishable realm.
At the beginning of time, God created a group of angels and lower gods. Twelve angels were willed to "come into being [to] rule over chaos and the [underworld]". The angels of creation were tasked with creating a physical body for Adamas, which became known as the first man Adam. Gradually, humanity began to forget its divine origins and some of Adam's descendants (Cain and Abel) became embroiled in the world's first murder. Many humans came to think that the imperfect physical universe was the totality of creation, losing their knowledge of God and the imperishable realm.
Jesus was sent as the Son of the true God, not of one of the lesser gods. His mission was to show that salvation lies in connecting with the God within the man. Through embracing the internal God, the man can then return to the imperishable realm.
Eleven of the disciples Jesus chose to spread his message misunderstood the central tenets of his teaching. They were obsessed with the physical world of the senses. The author says that they continued to practise religious animal sacrifice, which pleased the lower gods but did not help to foster a connection with the true God. They wrongly taught that those martyred in the name of Christ would be bodily resurrected.
In contrast, Jesus is able to teach Judas the true meaning of his life, ministry and death. Mankind can be divided into two races, or groups. Those who are furnished with the immortal soul, like Judas, can come to know the God within and enter the imperishable realm when they die. Those who belong to the same generation of the other eleven disciples cannot enter the realm of God and will die both spiritually and physically at the end of their lives. As practices that are intertwined with the physical world, animal sacrifice and a communion ceremony centered around "cannibalism" (the consumption of Jesus' flesh and blood) are condemned as abhorrent. The other Gospels say that Jesus had to die in order to atone for the sins of humanity. The author of Judas expresses the view that this sort of substitutionary justice pleases the lower gods and angels. The true God is gracious and thus does not demand any sacrifice. (Source)
We also see that it contradicts the old testament. It says the opposite of what the collection of books that have become the Bible states. It teaches that God did not create man as a being with flesh and blood. It states that we are not to be resurrected in the flesh like we read both in Daniel 12:2 and Romans 6:5 etc etc. The gospel of Judas is not a lost book. It was left out of the cannon for one reason. The claims it makes are lies and the early church recognized it for what it is. The book was written solely to support Gnostic teachings.Having been made almost two-hundred years after both Judas and Jesus life-time and any contemporary they have nothing to tell us about Jesus that is factual.
Resources
The Gospel of Judas
The Authenticity of the Gospel of Judas
What is the Gospel of Judas?
More in-depth evaluation of the book:
The Fictitious Gospel of Judas and Its Sensational Promotion