Actually the unifying mythos of Gnosticism was that you could, through a sequence of secret ritual initiations and coordinated instructions and practices, have a miraculous experience in which you would experience directly the reality of the underlying mythos of the group.
Upon completing the cycle of initiations and instructions and practices, you would then be able to unite your soul to the essence of the group's patron godling and thus your soul would be able to enter into the god's reality upon death. This transforming “knowledge” (gnosis in Greek) is the single reliably shared element of ancient gnosticism.
This mystery religion schema underlies most of the practices of the Hermetic Orders to this very day. The more prominent strains were heavily influenced by the Mithraic and Zoroastrian cults of Persia. This is the origin of the line that the Old Testament God was evil and Jesus was the Good God or messenger. Or that the world was the creation of a demiurge to trap the Divine Sparks released in the beginning from returning home.
Interestingly enough, it is also the origin of the idea that sex is evil and that woman, as the embodiment of sexual desire and passion, is inherently corrupting too. St. Paul, the guy who gets unfairly blamed for that thread, actually taught that marriage was God's greatest gift to man.
This schema was obviously quite adaptable to Christian ideas and phrases, and thus many Christian-gnostic groups appeared. They influenced the practices and ideas of the Eastern Christian groups rather heavily, including an extreme asceticism. One survival of Gnostic Influence in those congregations is the iconostasis, a partition covered by holy pictures between the congregation and the sanctuary. In Western Churches, this was torn down but remains in the altar rail.
The most successful Gnostic denomination was Manichean, and it is usually the Manichean strain of gnosticism, which later flowered in the Albigensian movement of the Cathars, which is the catch-all when literary critics are talking about “gnostic elements” in someone's style.