"So, when it comes to a definitive definition (and, I agree, definitive definitions are the best kind), I must remain highly agnostic."
I'm dying!
This might be a few aeons late, but. There was a SUMMIT held in Greece a few years ago that tried to declare the correct definitions of these words. They decided thus: Gnosticism refers to specific religious practices, such as those taught by Valentinus, etc., as Jim says, 'the formal, structured set of highly precise Eastern Mediterranean religious from two millennia ago' - this is Gnosticism, as a period of thought, as a philosophy, as a religion of some kind. Gnostic, however, refers to a type of religion or 'religious experience' that is characterized as a personal grappling with the heavens, or suchlike, in order to attain absolute knowledge. Essentially, personal mysticism. So, that encompasses everything from Evelyn Underhill to any Swami that ever lived, to Timothy Leary, to our own Thelema.
I usually go with: an emphasis on 'gnosis' & a belief in Sophia (not necessarily as deity). So, the Russian Christian Existentialists were very gnostic, Thelema is very gnostic, William Blake was very gnostic (see Blake's Visionary Universe), even Eastern Orthodox Christianity is very gnostic, though they'd consider that a slur.