The connection of that Serpent (of John 3:14) with the Serpent of Genesis is raised implicitly by Justin:
And the physiological discussion concerning the Son of God in the Timæus of Plato, where he says, He placed him crosswise in the universe, he borrowed in like manner from Moses; for in the writings of Moses it is related how at that time, when the Israelites went out of Egypt and were in the wilderness, they fell in with poisonous beasts, both vipers and asps, and every kind of serpent, which slew the people; and that Moses, by the inspiration and influence of God, took brass, and made it into the figure of a cross, and set it in the holy tabernacle, and said to the people, If you look to this figure, and believe, you shall be saved thereby. Numbers 21:8 And when this was done, it is recorded that the serpents died, and it is handed down that the people thus escaped death. Which things Plato reading, and not accurately understanding, and not apprehending that it was the figure of the cross, but taking it to be a placing crosswise, he said that the power next to the first God was placed crosswise in the universe.
http://www.newadvent.org/fathers/0126.htm
According to Justin, Plato heard from Moses the story of a crucified serpent and, without understanding that story, Plato wrote that
the creator of the world (=
"he") placed the Son of God crosswise in the universe.
But we know already that
the creator of the world was the evil demiurge for the Gnostics.
Hence the Gnostics would have agreed perfectly with Justin, here:
the demiurge crucified the Son of God (=not the god of the Jews) crosswise in the universe. When? At the origin of the world.
And what is allegory of
that Son of God? The Serpent raised by Moses.
But what Serpent was at the origin of the world, for the Gnostics? The Serpent of Genesis.
Hence the Demiurge crucified
that Serpent (=Jesus).
Hence the Gospel Jesus is the historicized Serpent of Genesis (for the Gnostics).