Peace be with you,
As you said, the Gnostic (that predate Christianity) believed the same as what is propagated by the Quran, and along comes a belief formed at a later date that contradicts the earlier belief (and the words of the Qur'an) and has somehow becomes the 'new reality' when actually is false.
Let me address this one point so as to avoid any confusion. When I said that Gnosticism predates Christianity, I meant that it predates Jesus himself. Some of it's followers incorporated Christian elements into their teachings and then began proposing that they possessed the true teachings of Jesus, teachings which explicitly contradicted what Jesus was known to have taught.
They believed for example that the God of the Old Testament (i.e. the God of Abraham, Moses, David, etc) was actually a
different god from the God of the New Testament. In fact, they believed the Old Testament God was an evil deity because he created matter, which in their worldview imprisoned the soul and was therefore evil. They believed in a cosmic battle between the Evil God who created the world, and the Good God ("The Father") who is attempting to liberate people spiritually, and the solution the Gnostics proposed was contained in the name of their movement, "Gnosis", or knowledge (specifically the so called secret knowledge they claimed to possess) was necessary for liberation. Jesus was seen as one of many manifestations of God, almost akin to a Hindu avatar, with the specific exception that he did not bear flesh and was pure spirit, and consequently why his crucifixion was an illusion.
Now it's rather interesting to note that the big emphasis among many early Christian writers was that Jesus possessed flesh and that his suffering was real. Here is one example from Saint Ignatius of Antioch (c 110 AD), who was the third leader after St Peter the Apostle in Antioch, one of the earliest Christian communities, and himself a disciple of St John the Apostle. He was gloriously martyred in Rome by being fed to beasts (his iconography depicts him being mauled by lions) but prior to his martyrdom, he wrote several letters to the existing Christian communities of his time, and in these letters he often attacked the errors of Gnostic teaching. I point this out because it has some relevance to the discussion at hand:
"Stop your ears, therefore, when any one speaks to you at variance with Jesus Christ, who was descended from David, and was also of Mary; who was truly born, and ate and drank. He was truly persecuted under Pontius Pilate; He was truly crucified, and [truly] died, in the sight of beings in heaven, and on earth, and under the earth. He was also truly raised from the dead, His Father quickening Him, even as after the same manner His Father will so raise up us who believe in Him by Christ Jesus, apart from whom we do not possess the true life.
"But if, as some that are without God, that is, the unbelieving, say, that He only seemed to suffer (they themselves only seeming to exist), then why am I in bonds? Why do I long to be exposed to the wild beasts? Do I therefore die in vain? Am I not then guilty of falsehood against [the cross of] the Lord?
"Flee, therefore, those evil offshoots [of Satan], which produce death-bearing fruit, whereof if any one tastes, he instantly dies. For these men are not the planting of the Father. For if they were, they would appear as branches of the cross, and their fruit would be incorruptible. By it He calls you through His passion, as being His members. The head, therefore, cannot be born by itself, without its members; God, who is [the Saviour] Himself, having promised their union.
Epistle of Ignatius to the Trallians, sections 9-11, c 110 AD
http://www.newadvent.org/fathers/0106.htm
I hope it is becoming ever more clear why the Quran supporting the Gnostic concept of an illusory crucifixion is as perplexing as it is problematic. Though once again, it is unclear to me as to how the verse in question is actually interpreted, and I would like to have a proper Islamic understanding of it (that is, I don't want to approach the verse with an preconceived notion, and want to understand it as Muhammad and his followers would have understood it.)
Peace be with you all