Gnosis are entities shrouded in mystery with a definite hostility toward mankind. The Gnosis exist in a unique dimension separate from the physical plane (or real space) called the imaginary space; therefore, it is normally impossible to come into contact with them. Strangely, despite their destructive powers and obvious sentience, they are mostly comprised of sodium chloride (pure salt); when translucent, they are composed of water and sodium hydroxide.
If someone is touched by a Gnosis, they either die by breaking down into salt, or become Gnosis themselves. The latter of which occurs if one is touched but survives. However, Shion Uzuki was touched by a Gnosis and failed to turn into a Gnosis. This most likely comes from the Gnostic belief that in the quest for enlightenment, three things can happen: One can be unworthy, and one just dies (turning into salt.) One's soul can be considered worthy, but one's material body is still impure (turning into a Gnosis.) Or one can be of the few who reach enlightenment without losing one's body (Shion surviving being attacked by a Gnosis). In the story of Lot in the Bible, his wife is turned into a pillar of salt after looking back on the city of Sodom.
Gnosis are human wills who resonate passively with Anima to the point of seeking dispersal and destruction. Because they fear and reject other people, they reject the entire collective consciousness of humanity. People who become infected with this fear by contact with Gnosis may also become a Gnosis themselves if they submit to the fear or try to escape from it. People who resonate purely with Anima do not become Gnosis, which may be why Shion never turns into a Gnosis after being attacked and almost whitened.
Gnosis come from imaginary space, which occupies the same space as real space (normal space). The collective consciousness of the Lower Domain (the universe of human consciousness) is composed of both the real (physical) and the imaginary (spiritual) sides. What looks human on one side looks like a Gnosis on the other side. Everything that exists in the real world also exists in the imaginary world, which is why there are even Gnosis planets, such as Cathedral Ship. Although the term Gnosis generally applies to the wills that seek dispersal, everything that exists on the imaginary side is composed the same as a Gnosis. This is also why Abel's Ark is seen as a giant Gnosis, yet at the same time it is not a Gnosis.
According to Jr., Gnosis have been reported throughout history, albeit in very few sightings and nothing near the scale at which they currently appear. But it was during the Miltian Conflict that the Gnosis came into our dimension en masse. It was this event that caused the dispersal phenomenon that was destroying the universe to rapidly advance.
By activating the Hilbert Effect, Gnosis within the radius of the effect are drawn into the same physical plane as humans, allowing them to be affected by conventional weapons. Due to the variations in their external appearance, the standard practice is to categorize and name Gnosis after mythical, legendary, and folkloric creatures. Examples of these are Goblins (top-heavy white bipeds), Golems (gangly humanoids with flexible arms), Manticores (large greenish caterpillars) and Gremlins (fishlike beings, one of which merged with an A.G.W.S. in Episode I as a boss on the Woglinde). Other common types include boxlike whales and amber-skinned seals.
Gnosis also appear to have the ability to possess machinery and integrate it into their body, thus giving themselves extra abilities based on the machinery they possess. A Gnosis of the "Gremlin" type possessed an A.G.W.S. unit and was able to use its weapons.
In Episode III, four Gnosis appear on Abel's Ark, guarding the way to Dimitri Yuriev's location and Ω Res Novae, similar to the four pillars of Deus' core in Xenogears. These each resemble several of the Omnigears from Xenogears, and are named Natus Flamma (resembling Bart Fatima's Andvari), Natus Glacies (Resembling Citan Uzuki's Fenrir), Natus Lumen (Resembling Billy Lee Black's Renmazuo), and Natus Tellus (Resembling Rico Banderas' El Stier).
During the course of Episode III, it is revealed that the Gnosis are in fact human spirits that refused to join the collective unconsciousness when they died.
Gnosis is the Greek word for "knowledge." The games are referring to Gnosticism, a system of beliefs which teaches that the material world is evil, and only through the pursuit of the knowledge of the true nature of the world can man return to the spiritual realm after death.