A lot of the things you first think of when you think of Gnosticism are merely cosmetic elements that were not universal (not even all Gnostics were pretending to be Christian).
It is not actually required for Gnostics to view the Yahweh of the Old Testament as an Evil Demiurge, or that there are other gods above him. Nor to make the Serpent good. But such things certainly were and even still are common.
It's not too hard to turn Biblical Christianity into Gnosticism with only a few key heresies. The Greek New Testament uses many of the same key words and phrases the Gnostics later used. Just like modern JWs and Mormons and other cults uses the same terminology Evangelicals use, but they mean different things by them.
Logos and Sophia are used largely because they are accurate Greek translations of Old Testament concepts, The Word and Wisdom. Both the terms "Archon of the Kosmos" and "God of this World" used of Satan by Jesus in John and Paul respectively, are used in Gnosticism for the Ialdobath.
The core tenet of Gnosticism is disdain for the physical world. The Bible tells us the Earthly realm has been corrupted by The Fall. But it will be Restored in the End in Revelation 21-22. And those who are Saved will be perfected in The Resurrection.
Gnosticism however views the Physical world as inherently Evil, and our Physical bodies as a prison. So denying a literal Bodily Resurrection of all Believers is key to them.
This is of course what Amillenial and Full Preterist eschatology requires. Denying a literal Resurrection.
One Preterist website I had studied talks about how the Resurrection is only spiritual, it's about freeing the Souls of the Saved from Sheol only.
And they tied into that a belief in either The Gap Theory or the Extended Day Theory, I don't recall which one. They insisted physical Death was always Adam's destiny, the Fall only caused spiritual death. And some Christians even consider theories that Adam didn't have a physical Body before the Fall. That idea is refuted to me by him saying Eve was made of his Flesh and Bone, but I do think it's possible Blood (as we currently define it) didn't exist before The Fall.
Augustine of Hippo was both the key popularizer of Amillenial eschatology within the Church, and one of the first to reject a literal interpretation of Genesis. He came out of Gnosticism, and was open about still revering the Platonic Philosophy from which Gnosticism was derived.
I should add of course you can be Futurist and Premillennial and still hold a Gnostic outlook, if you Believe the Resurrection is only Spiritual, which I have seen examples of.
It's funny how Dan Brown in The DaVincci Code sought to create a narrative of modern Roman Catholicism and the Gnostics as arch rivals. Brown did not comprehend Gnosticism at all, his version of them was the opposite on this very key defining trait. (The hardcore ancient Gnostics denied Jesus had a physical body, so their Jesus certainly didn't have sex or have children). The Vatican meanwhile is pretty Gnostic thanks to how much of Augustine they canonized.