I don't understand why you are singling out Gnosticism from other heresies. Expand or reframe your question.
Gnosticism, in general, is "salvation through knowledge".
Well, it's a bit more than that actually. Defining "Gnosticism" that way is neither meaningful nor useful- every religion can be called "gnosticism" by that definition.
I would say that someone has slipped into Gnosticism, if, for example, he believe that the material world is a prison for souls created by a demiurge, and that we need to to escape materiality through esoteric knowledge and thereby merge into the Godhead.
is different, is acquired differently, and for a different purpose. So long as someone maintain Orthodox faith, in the life of the Church and its Mysteries, he is an Orthodox Christian and certainly no Gnostic.
I agree. The devil behind Gnosticism is the "cosmic cynicism" that goes with it, formulating the impulse "if I tough it out and be good, I can get out of here".
At the same time though, Gnosticism is "alleviated" by the correct knowledge. Hence, my wondering of the "line".
A Baptist, perhaps, would claim the only necessity is to accept Jesus as your savior. But if there is a need to accept a certain amount of truth more to this to understand and fully appreciate God, why is that not tip-toeing the line of "special knowledge" that will save you from Hell?