First of all, before I answer your first question, you have some facts about Gnosticism correct, but one thing is definitely wrong.  You say that according to the Gnostics, Jesus is a bad guy.  This is definitely false.  What the Gnostics taught is that YHWH–the God of the Old Testament was a “bad guy.”  To the Gnostics, Jesus was absolutely a good guy.  The Gospel of Judas is a perfect example of a document which has the Old Testament God being a bad God but Jesus a good guy.

I have written an article which answers this first question.  Here is the link:   https://evidenceforchristianity.org/judas-another-gospel/

Judas betrayed Jesus because he was disappointed that Jesus was not starting a rebellion against the Roman government to free the Jews.  He had the common Jewish misconception that the Messiah was going to be a political king who would save the Jews and return them to the political glory of the days of David.  I cannot prove that this was the reason, but I believe it is the most likely reason Judas betrayed Jesus.  It was probably not about the money.  When Jesus was arrested and crucified, Judas realized his error and, out of guilt, returned the money and hung himself.

Absolutely and without any doubt, no!  This is an outrageous and completely unfounded slur begun by people who were heretics or mortal enemies of Christianity.  This claim should be completely rejected by anyone who has any respect for truth.  There is not a shred of evidence that this happened, and everything we know about Jesus says it is ridiculous.  Jesus asked the crowd, “Can any of you prove me guilty of sin?  The crowd was speechless.  Jesus did not commit adultery with his followers.  Outrageous!

This is a mythical invention of Greek gnostics.  He is one of the lower-level emanations of the ineffable “god” of Gnosticism. You will find something about Saklas in the article I gave the link to above.

Read the article about the Gospel of Judas above.  It will answer your question.  This was a Greek philosophy/religion which, unfortunately, influenced some believers in Christ in the first through third centuries.  Gnostics believed that physical matter is bad, that the God of the Old Testament was an evil God, that Jesus was an emanation of God who merely occupied the body of the person we call Jesus.  This was heresy and was never accepted by the mainstream Christian Church.

The Gnostics had their heretical beliefs and they wrote documents to support their belief, such as the Gospel of Judas, the Gospel of Thomas, the Gospel of Mary and many more.  Like anyone who has a strong opinion, naturally the Gnostics wrote literature to support their beliefs.  The Gnostics did not deny most of the facts of the New Testament.  They did not deny the miracles, the miraculous birth or even the resurrection.  The things they claimed are not a matter of historical evidence so much, but are a gross misinterpretation of what Jesus and the mainstream church taught. They are bad theology more than ignorance of the historical evidence.

What do you mean worry?  Feel free to read the Gospel of Thomas or the Gospel of Philip. There is nothing scary about them.  But they definitely are not Christian.  They are blatantly influenced by Gnosticism.  They were written in the second century, and are not an accurate reflection of what Jesus did or taught.  You should read these supposed “gospels” and decide for yourself.

Here is an article about the Gospel of Thomas.   https://evidenceforchristianity.org/who-wrote-the-gospel-of-thomas-and-should-it-be-in-the-bibler/

Gnosticism is a heretical aberration of Christianity.  It is helpful to understand Gnosticism so we can recognize those who move in this direction today, so we can defend biblical Christianity against this heretical philosophy.  There is nothing to fear here.  Your job is to understand and be prepared to answer.

John Oakes