Gnosis and Gnosticism are not necessarily always the same thing. It is taught that there have indeed been Magi (Priests) who taught about Gnosis under the name Gnosticism; however, it is also taught that Gnosis is not limited to Gnosticism, because Gnosis is the Root Wisdom of all authentic Religions, and is why Samael Aun Weor teaches about Gnosis using Buddhist, Hindu, Islamic, etc. terminologies as well.
Nonetheless, Samael Aun Weor also taught about and commented on that which is called Gnosticism, and the following thread in another forum has compiled references to just about everything publicly available by Samael Aun Weor on the topic:
Gnosis, Samael, the Zohar, the Bible, and Gnostic Scriptures