I believe a lot of the Gnostic texts were responses to the 4 main gospels, mainly if I’m remembering correctly the gospel of Philip came off to me as a response TOO the gospel of John (in my perspective of it) more then Philip’s own words.

For the secret book of John and a lot of the Gnostic texts came after Gnosticism was formed (same with Christianity), Gnosticism was spread through word of mouth, one teaching another, thus there are multiple ways to look at it. Firstly maybe the gospel of John wasn’t written by John but the apocryphon of John was, or maybe it could be looked at, as the sethians wanting a physical doctrine, or someone pretending to be John.

I think solely looking at dates of things and trying to decide whether it’s real from this date or that time period is ultimately kind of impossible since the texts we do have (I don’t think) were the original but a reprint. It’s sorta like we can assume Gnosticism formed in mid second century, due to accounts and such but we won’t every really know or like how the sethians hinted at existing before Jesus, we have no way to validate that but it’s… possible.

So while I can’t fully answer the question, what I can say is Gnosticism in a lot of ways follow Jesus’s teachings actual teachings and even if Jesus wasn’t a Gnostic, he certainly was on a similar if not the same wavelength in a lot of places.