For starters, one could just say that gnosis is to nous as perception is to the senses, or reason to the rational mind. Gnosis is what the nous "does" or "receives" -- the nous essentially being our organ of spiritual perception and discernment (to paraphrase Aristotle, it is that part of us that is divine, or at least closest to it).
If we didn't have this faculty -- or it us -- there would be no realistic way to discern spirits, to read the signs of the times, to deeply understand scripture, or to even operate this bus, really. In its absence we are spiritually autistic, as it were. And we all know spiritually autistic people. They pretty much run the show, don't they?
A related problem is when the nous is appropriated by lower regions. For example, many self-styled gurus and spiritual hucksters presumably have an activated nous, except it's merged with the ego (or lower), which drags it into the mud. That pattern is similar to, say, a psychoanalyst who has valid knowledge of the unconscious mind, but who lacks insight into his own motivations, and ends up seducing a patient.
But for clarity of thought in this upper atmasphere of the soul, I don't think it's possible to surpass Schuon, so let's see what he has to say. Conveniently, one of his books is called Gnosis, with the subtitle Divine Wisdom. So gnosis is divine wisdom, in contrast to, say worldly knowledge.
And if that is too saturated, you can always just say (n) vs. (k) to distinguish the one from the other. But in some form or fashion one must make this distinction, because if one doesn't, one becomes either an atheist or literalist -- which amount to the same thing, since both apply the wrong category of thought to the spiritual dimension. It's a ubiquitous problem, for "Men tend not to live on anything but the ground floor of their souls" (Nicolás Gómez Dávila).
More from Dávila: To be stupid is to believe that it is possible to take a photograph of the place of which the poet sang. To put it another way, religion can surely be understood rationally, but not only rationally.
Rather, one also needs an uncorrupted imagination, which "is the capacity to perceive, through the senses, the attributes of the object which the senses do not perceive" (NGD). Thus, "The imagination is not the site where reality is falsified, but where it is fulfilled" (ibid.).
For "When things appear to us to be only what they appear to be, soon they appear to be even less" (ibid.). For example, "The Gospels, in the hands of the progressive clergy, degenerate into a compilation of trivial ethical teachings" (ibid.). But in reality, "Faith is not assent to concepts, but a sudden splendor that knocks us down" (ibid).
Back to Schuon. The lone reviewer pretty much says what we just did, that gnosis represents "the highest aspect of the Intellect -- and is not to be confused with the lower faculty of rational thought [while not negating it, I might add]. Gnosis deals with intuitive contact with the higher level of the Self, while mere rational thought deals with the strictly human level/profane intelligence for a profane world."
"Gnosis," according to Schuon, "refers to supra-rational, and thus purely intellective, knowledge of metacosmic realities. Now this knowledge cannot be reduced to the Gnosticism of history," nor can it "be held responsible for every association of ideas or every abuse of terminology." As is true of any religious concept -- or any concept, frankly -- improper abuse does not negate its proper use.
That's right, what Bob said: "the fact that an imposture necessarily imitates a good, since otherwise it could not even exist, does not authorize charging this good with all the sins of the imitation" (Schuon). Might as well condemn religion because of Deepak, for "To claim that all gnosis is false because of Gnosticism, amounts to saying, by analogy, that all prophets are false because there are false prophets" (ibid.).
Nevertheless, "It is a fact that too many authors -- we would almost say: general opinion -- attribute to gnosis what is proper to Gnosticism and to other counterfeits of the sophia perennis, and moreover make no distinction between the latter and the most freakish movements, such as spiritualism, theosophism and the pseudo-esoterisms that saw the light of day in the twentieth century" (ibid.).
So, what is Gnosticism? Pretty much "a fabric of more or less disordered speculations, often of Manichean origin; and it is a mythomania characterized by a dangerous mixture of exoteric and esoteric concepts." I'm sure you're all familiar with the parable of the ice cream and the dog doo.
So, when does gnosis go off the rails? I can think of several pitfalls, pride being foremost among them. There is also the danger alluded to above, of conflation with an unredeemed imagination, or with one's own mind parasites. For this and other reasons, it is always best to channel the of flow gnosis -- or O --> (n) -- between the banks of an institutional or traditional river. It's the difference between, say, a Meister Eckhart vs. a William Blake or Emanuel Swedenborg, the latter two being too much a mixture of dream and reality.