This question was tossed around a while back.
What do Artists do? If you answer, “creates Art,” all that you have done is change the question to, “what is Art?” Likewise the question, “What do Gnostics do?” is a different way to re-frame the basic definition question asked of Gnostics, “what is Gnosis?” These questions are ill-formed because they include an error of category.
Art is one of the areas where 'knowing' is of the gnosis variety. That is why it is something of a holy grail for epistemelogically minded folks, and a source of endless failure.
What does music taste like? What color are ideas? These are not meaningless questions, they make a kind of sense, but when we play across categories like this we expect indications or references, we don't expect the one to really be expressed in terms of the other. Likewise, if you ask for something in one category of knowledge, gnosis, in terms of the other category of knowledge, episteme, don't expect definitions, indications and references are all you are going to get. If it were episteme, it wouldn't be gnosis.
The general assumption of our culture is that all things that are 'real' knowledge can be put into terms of episteme. A Philosophy of Science professor of mine once pointed out with derision that there were some number of different 'definitions' of paradigm in the classic The Structure of Scientific Revolutions, not considering that they weren't definitions at all, but rather, descriptions. If I give different descriptions of Joe, is he an ill-defined concept? Well, he would be if he were a concept.
The question, “What do Gnostics do?” has two potential answers: the obvious pragmatic answer is “What Gnostics do consists of their actions,” the other answer would include a definition not only of Gnosis, but of the means of attaining it, which as we have seen is an error of category. Ah, it takes you right back to Classical Athens, doesn't it? That old gadfly Socrates pointing out the hard way that this sort of thing just doesn't work.
But let's not dismiss the question just yet. I think the reason it comes up occasionally is the koan-like quality of the unanswerable question, the question that asks its nature. The real question is: what am I, a Gnostic, to do in regards to Gnosis? The answer cannot be in episteme form. No definition is possible, living consciously means grappling with it as you go along. Let's also not dismiss the pragmatic answer, when looking for the answer in your own life, looking at what other Gnostics find to be of essential value in their lives is the best guide you will find outside of yourself.