Gnosticism was a rejection of the Yahwey male god after 70AD as he was seen to be defeated by Jupiter. Alexandria Egypt was the New York City and had been building up its libraries and culture for three hundred years and at the point the first Gnostic sects we see appear there in the large Judean populations. Christianity forms out of this same population and is why Jesus lived Egypt as a child in the gospels. Of course some of this was coming from the Northern enemy Samaritan peoples and Judeans themselves who were BOTH heavily invested in Zoroastrian-Mythriac magic since Persia first built the Temple under Ezra, their agent in the field that gathered up random tribes into one Judea from conquered kingdoms of the Empire.

The council of Nicea did not outlaw Gnosticism totally. They had many original pagan groups at the Council and they organized all religions into a Universal (trans: "Catholic) Cult. St. Augustine was a pagan at the council and converted because of it.
The Arian Christians survived until the 589AD in Spain (...even after Saint Nick bitch slapped their founder at the Council of Nicaea for denying the trinity). It was also the religion of the Goths that sacked Rome and St. Jerome was told to write the Catholic Vulgate in Latin to educate their new Goth Arian overlords.

So Gnosticism was always intertwined in all three Bible faiths and even Communism through Kabbalah. It reapers from time to time and basically helped purge pagan beliefs, replacing them with Middle-Platonic beliefs from the perspective of an Alexandrian Judean or Christian.