In Chaos Magick, gnosis is a lapse in conscious/rational/discursive/ discriminatory mind that allows the subconscious "magickal mind" to fully function. The easiest example of this state that comes to mind is the exact moment of orgasm. The term "gnosis" is used (I think) because the state allows for the instantaneous, intuitive (as opposed to consciously deduced) grasping of a much larger set of the variables involved in causing an outcome to manifest from any given state of events.
This is done because the time it takes to reason out such a large set prohibits effect; by the time the relationships are consciously worked out, the variables have changed. It must be done instantaneously in order to be effective. Were there to be a qabalistic tree containing correspondences for Chaos Magick, gnosis would likely be placed at Kether, and the psychic censor (the most basic, binary discriminatory function) at Chokmah. After the state of gnosis has ended, one can get an idea of how "profound" the state was by trying to recall exactly what happened while you were in in it. The less that can be remembered, the more profound the state. Perfect gnosis would leave no signature in the rational mind, not even the awareness of a gap in time.