Having consulted a few various sources ( books, Web resources, talks with both experts and informed enthusiasts ), there emerged a picture curiously similar to your original definition of Gnosis.  Briefly, some other points, especially regarding relation to the cosmos and the eschatological or "evolutionary" questions, can be subsumed under single differentia specifica not necessarily shared with multiplicity of orthodoxies and heterodoxies.  Just a reference to a landmark book: beautiful Happold's anthology, "Mysticism". He differs between three kinds of mysticism:

God-centered ( all "classic" religious paths ) Nature-centered ( more "poetic" expressions ( Blake, Whitman, .., also - classical Paganism, naturalistic Taoism and many others ). Soul-centered ( in this sense, "Soul" has more similarities with "Self" ). The immortal man, Divine Self, soul spark, the deepest "I" - this is the true goal of the third way.

In this respect, the widest possible definition ( or intuitive flash of insight into the nature of Gnosis, as liberated from frequently more nuanced, but, alas, simultaneously musty and sometimes anemic academic dissections ) is: Gnosis is awakening to the deepest Self and Light within. It is the cornerstone of dramatic transmutations of consciousness achieved through theoria, death-and-resurrection initiations and spontaneous awakenings. It is pilgrimage of the self to the Self.

So, Gnosis in the widest sense is the "discovery" of inner Self as the true "I". A corollary must be added: not all meditative/contemplative disciplines can be termed as "Gnostic". Only those that affirm the reality of Reality qualify ( this adjunct dumps Advaita and Ch'an/Zen ).  The premier examples of Gnosis are Shamanism, Hermetism, Christian and Jewish Gnosticisms, Kashmir Shaivism and Siddha Yoga in general; also, some schools of Vajrayana/Tantric Buddhism ( six Yogas of Naropa- here, things become fuzzier due to the Mahayana Buddhist dogma of illusory nature of the Cosmos ).  In short, Gnosis equals radical self-reliance on and/or fierce transmutational/expansionary voyage towards Inner Self/Light within.

Arvan Harvat