One of the main reasons that I wrote the piece On the Limitations of Make Believe is to try to break open the traditional theological box. (Let's try “traditional theology” as the term for the discipline instead of simply thinking or reasoning about religious things, rather than my previous “non-trivial theology” and see if that helps with confusion.) Some may find this obstructionist. Yet, I've stated elsewhere that if someone wants to satisfy themselves that a traditional theological definition of Gnosticism is a logical impossibility, there is the wall and you know where your head is—start banging until you are satisfied.
There is an assumption that for someone to be able to use a definition in an academic setting, that it has to be a traditional theological definition. That is to say, a definition couched in the framework of the tradition of Christian theology. Yet, that would mean leaving Gnosis out of it, and frankly I've yet to see a theological term applied to Gnosticism that makes sense except as a poor analogy. Traditional theology did not grow out of a Gnostic milieu, to say the least. It's focus has always been upon doctrine, in a framework where the view is that doctrine determines beliefs.
One of the big fundamental issues is that almost (if not) all of traditional theology developed within a model of religion that grew out of the historical formation of Christian Orthodoxy. This focused on uniformity of doctrine, and beliefs. And really has a kind of “top-down” way of viewing things. You start with the assumption that the source of belief is doctrine, or with the assumption that these things you are considering exist and can be described in an authoritative way. This gives one the odd result of an “objective” religion, one without people.
Gnosticism by the very nature of Gnosis, has more of a “bottom-up” approach. Gnosis is individual, it cannot be removed from the subject having it. This leads to expressions of that Gnosis that include beliefs. It also leads to a very modified “top-down” model of someone learning about these beliefs and such, and then seeking Gnosis.
To seek a traditional theological definition, “Gnosticism,” one would need to enter into that framework, leaving the people (the subjective) behind, while forcing it into the mold of a doctrinal religion. Sure we can limit the doctrine to a small set, but that is what we will get—Gnosticism equals doctrine. Let's just say that a “Gnosticism” that doesn't involve Gnosis or Gnostics, is a bit of a problem.
Is this quest for a traditional theological definition the only approach? No. And I don't understand why this is assumed to be the case. It is far from the only form of human endeavor that can live up to “academic acceptability.” In fact, it doesn't exactly have a sterling reputation among the academic disciplines.
While an academically acceptable definition truly comes down to the academicians who accept it, we can look at it in broader terms. The goal of such a definition is to limit the scope of the study, and to give meaning to that scope.
The first comes down to scope: what is inside the scope and what is outside; what it is that is studied. This is always fuzzy on the level of details, but a general consensus is required. We must remember that the definition actually comes after the consensus. It is always an inadequate attempt to formalize what those involved recognize. We can, hopefully, recognize in this the issue of trying to put gnosis into the realm of episteme. The lack of awareness of this issue is a larger issue in academia, leading to lots of cracked skulls and dented walls. Apparently, the study of religion was struggling with a rather clumsy definition that involved “superhuman beings,” for example. This lead to some mental acrobatics to get Buddhism to fit the mold (or gnosis to fit episteme.) (Buddhism and the definition of religion.)
The second issue is one of the scope being meaningful to study. By which I mean, that the conclusions about the scope or concerning relationships within the scope are academically interesting and approachable. This would mean that even the study of a clearly defined thing, may be either of no interesting, or unapproachable by the methods of academia. The latter is an area of agnosia for most of academia, it doesn't recognize it exists. So, I think we can safely ignore it in our current considerations. This leaves us with the issue of being of academic interest.
One can clearly define any number of things that do not meet this criteria, they may serve as data point in some other interesting enterprise, but are not themselves interesting. The collection of lint in my dryer, for example, just isn't academically interesting, unless it is part of a larger study of dryers or textiles.
Applied to Gnosticism, we can see the first issue as: is this or that a part of Gnosticism? And, the second issue as: is “Gnosticism” a meaningful category? These are not unrelated, yet they are distinct approaches to the question: what is “Gnosticism”? What is its scope? And, what does it mean?
When approaching the formulation of a definition for academic use, we need to bear in mind that we can come up with a definition that isn't meaningful. I would argue that the traditional theological approach can only lead to that end.
Buddhist studies makes for the closest analogy, although still very different in history, the amount of material, and the number of continuous traditions. I found this review of the book Buddhist Theology interesting. It advocates a new and specifically Buddhist theological approach. Another approach is Engaged Buddhist Studies, however that seems to be a splitting off of practical from abstract issues.