The adjective gnostic describes something related to mysterious intellectual or spiritual knowledge. That gnostic text you've been trying to read is mystical and a little bit obscure.
The word gnostic comes from the Greek word gnostikos, meaning "knowing" or "able to discern." If the word has a capital "G" — Gnostic — it describes something related to Gnosticism, a form of nonconventional mystical Christianity that emerged in the 2nd century and is still considered heretical by many Christians. Used more broadly gnostic can describe something that has mystical knowledge, especially related to spirituality.