The more I think about it, the more convinced I am that there is an essential difference between mysticism and gnosis.
On the surface, both deal with the phenomenology of spiritual experience — direct encounter with the Divine Other, and/or the experience of Union with the Divine. But then there seems to be an essential ingredient that differentiates mysticism from gnosis, or vice versa.
Gnostic spirituality seems to imply that direct personal experience always trumps the received wisdom tradition (if the church tells me to serve the poor, but my own gnosis directs me not to interfere with the poor because they have their own spiritual karma to unravel, then as a gnostic I will ignore what the church says and follow my inner teachings). By contrast, conventional religion suggests that tradition always trumps experience — which is why the Catholic Church will only accept mystical teachings that are fully and completely consistent with official church teaching. Perhaps mysticism is the golden mean between these two positions. Mysticism dares to suggest that personal experience and received tradition must always inform, enlighten, critique, and shape each other. Gnosis and tradition are equally valuable, equally marvelous and miraculous arenas where the Holy Spirit can and does act. Likewise, both are subject to scrutiny — from each other. It is in this arena of mutual humility and shared vulnerability that God can do God’s mighty work of shaping us — and the community — in ever new ways.
I’m not sure if this is all that there is to understanding the difference between mysticism and gnosis. But it’s a start.