Gnosis is a direct, immediate knowledge of the Divine, through experiencing the Self of the Individual and realizing its role within the All.
We are each moving through our own sphere of the cosmos, interacting with the energy around us, surviving as a biological super organism which has created a neural map from biological receptors that translate this energy into a complex series of illusions that we perceive as "reality".
However, scientific discoveries in biology, quantum physics, and cosmology have allowed us to intellectually break down these patterns, and prove that most of what we perceive as "real" is only the result of our brains compiling information together in order to consolidate the trillions of living organisms that make our physical bodies up into one, single, super organism.
It's one thing to understand this rationally, but it's another to understand this intuitively, on a very real "spiritual" level.
Gnosis is an intuitive understanding of the Self as a bodiless, consciousness beyond the ego's perception of this false reality, and creates a transmutation within the mind that allows a deeper perception of the Self as part of the All.
To illustrate, picture each user on the Internet as part of the World Wide Web. Each user is individually a piece of the ocean of information that connects together to form an unbroken, complex map of a virtual world that stretches across the globe.
We all fall victim to our own perception of "reality", and it is easy to allow the ego to construct the false notion of real separation from the rest of the All. However, the quantum level of this "reality" shows a sea of gaseous particles made of energy that LITERALLY jumps in and out of existence.
Atomically, we are all connected. Spiritually, we are each capable of intuitively understanding and experiencing this connection.
It is this intuitive knowledge, this gnosis, that sets us free. This gnosis of Self is what allows each of us to KNOW the very nature of the Divine, which is the culmination of the All.