With something so central to Gnosticism as Gnosis, it is important to say what we mean and don't mean by it. And since it is much easier, and will narrow the field of discussion greatly, I will begin by discussing what Gnosis is not.
Gnosis is not withheld information. This was the accusation of the early heresiologists starting with Ireneaus writing against the “so-called knowers.” It is the “I've got a secret” view of Gnosticism, as a needlessly elitist, “we could tell you but we won't” group who's knowledge is entirely limited to easily sharable information. This is still a common misunderstanding among those who are not Gnostics, and cannot see any difference between knowledge and information.
Gnosis is not whatever someone wants it to be. This is the take of the use of Gnosticism in the political writings of Vogelin. In this view the Gnostic is like an older taller child holding the ball up out of the younger shorter children's reach. Instead of playing “I've got a secret” they are playing “you can't argue with my position, I'm special.” This view would put Gnosis under the same aegis as revelation. There are modern pseudo-gnostic groups that one could replace “Gnosis” with “special revelation” in their documents and not change the meaning.
Gnosis is not revelation in the traditional sense. The claims of “Revealed Religions” is that one or more individuals received unique information from the Divine, that is universally applicable. We are familiar with the Bible, the Torah, and the Koran, of the fundamentalists: these are seen as “revealed truth,” information given by God in that same form. The weakness in this view is that people always show up claiming to be a newer prophet, and so a rigid hierarchy is needed to control what information is and is not “revealed truth.” Other than the fact that they are more than willing to share this information by force if necessary, it is the very same view of religion that the early Gnostics were misunderstood and accused of having, it is just that they were seen as having revelation experiences outside of the control of the hierarchy.
Gnosis is not whatever we want it to be. This is a trap to which the modern mindset is particularly prone. In this world of oppression, when they hear of Gnosticism, many hear foremost the social and religious liberation to be had. They fall in love with those ideas, and do not see deeper—that they are expressions of the pursuit of ultimate liberation. They see themselves free from oppressive hierarchies, and perhaps for the first time, feel free to have spiritual experiences and relate them to others, but they have not matured into a Gnostic understanding. Therefore they can fall into the trap of many a mainstream Believer, jumping from a few spiritual experiences to vast conclusions that are psychological in nature. Something that assuages the psychological needs of the personality or the ego is certainly not Gnosis.
Gnosis is not any kind of information. While we might say that scripture, for example, contains Gnosis--it is a manner of speaking, a metaphor. What scripture contains is far greater, it contains means of evoking Gnosis in us. Gnosis is not contained in books, although it can be found in books through our interaction with them. I am not suggesting we stop speaking in metaphors, or replace “Gnosis” in such contexts with “things that can evoke Gnosis.” But when we speak of Gnosis itself we must be clear. If we are not clear about this central concept in our tradition, how can we hope to be of service in helping as many as will seek Gnosis? Gnosis is not found in books, it is only found in people. It cannot be directly transmitted only evoked. We are heirs of a rich tradition, but that tradition is not Gnosis, it is a path to Gnosis.