The word can be used both narrowly and loosely, yes. There was even an attempt, many years ago, to adopt a quasi-official academic definition of the term at a conference in Messina (Google it if you're interested). That's outmoded now, but I think that the concept is coherent enough to be defined with reasonably meaningful boundaries. Denzey Lewis is gratuitously rejecting this position (and, to be fair, she's not the only one).

In its loose sense, "gnosticism" is basically used to mean "weird spiritual stuff", or at the most "weird spiritual stuff which involves rejecting the world and believing in lots of spirits and demons". It has overtones of secret knowledge, too.

I certainly can't comment with the gravitas of a pivotal authority in UFOlogy, however.

Consider it one of the latest American religious movements. A huge chunk of the UFO movement has encapsulated old practices and ideas of the spiritualists and theosophists. Slap on a thin coat of science fiction and Fortean paints and you're good to go.

Ah, yes. That has been a fashionable strategy for a while. It caught my fancy in grad school. But extreme skepticism provides very little payoff in the long run. I have no dog in this fight, but I tend to think Gnosticism was a real phenomenon. Really I am taken with the existence of many strands of ancient Christianity that fell by the wayside more or less. The Didache is fascinating. Origen’s fall from theological favor. Simon Magus. Adoption theology. So many fascinating byways of Christianity over the centuries.