The two are not the same.
The former is a declared heresy and is, for all intents and purposes, dead. The latter is a component of at least one orthodox Christian tradition, primarily Eastern Orthodoxy.
I find Gnosticism unappealing, yet I confess that gnosis enraptures me to the core. I am not Eastern Orthodox, and I’m sure that my definition of gnosis differs from Orthodox tradition and theology. All the same, there is some overlap.
The Orthodox Christian tradition describes gnosis as spiritual knowledge obtained via theosis. It is the knowledge only saints and mystics are likely to attain – the sort of knowledge that provides insights into the divine and the eternal rather than the mundane material or natural world.
I have sometimes seen gnosis described as mature or developed understanding based primarily on spiritual knowledge or – more precisely – on experiential and intuitive knowing. Put another way, gnosis does not emanate from books or doctrine but from living and discernment.
Gnosis is the knowledge of the mystic rather than the rational thinker. It comes not from external, supposedly authoritative sources but from inner thinking, experience, and contemplation.
Gnosis is the understanding that knowledge alone cannot lead to spiritual knowledge. To know spirit is to experience spirit. The Eastern Orthodox refer to this as noetic potential. There is nothing secret or hidden about the knowledge noetic potential can access.
Again, I am not Eastern Orthodox, but I admire the tradition’s inclusion of gnosis in its theology. However, I don’t believe it emphasizes gnosis enough. More to the point, I think gnosis has become a “first and necessary thing.”
Christians can rebuke Gnostics all they want, but they would do well not to aim their chastisements at gnosis or declare gnosis a sign of spiritual pride.
To know Christ is to experience Christ.
Spiritually. Personally. Directly.
All other knowledge is merely knowledge.