What the ancient teachers grouped under this name share, to the extent that they share anything, is a diagnosis. The human problem is not sin, and not disobedience, but ignorance. Human beings do not know what they are, where they came from or how to get back, and what saves is gnosis, a knowledge that is not information but recognition, the kind of knowing in which one wakes up rather than learns a fact.
The reason for the predicament is usually a defective creation. The visible world was not made by the highest god. Above and beyond it is an unknowable source, and around that source the pleroma, the fullness, containing ranked pairs of divine beings called aeons. Something goes wrong within the pleroma, in the Valentinian version, the aeon Sophia, wisdom, attempts something beyond her station and produces a misbegotten offspring outside it. That offspring is the demiurge, the craftsman who makes the material world, ignorant of what is above him and announcing that he is the only god. He is not usually depicted as evil so much as limited and arrogant, which is the more interesting version of the idea.
The human being is therefore a mixture: a divine spark or seed, of the same substance as the world above, encased in a body and a psyche belonging to the demiurge's world. The spark is dormant and must be woken. A revealer descends from the pleroma to wake it, passing through the ranks of hostile powers, the archons, who govern the planetary spheres and obstruct the soul's return. In Christian versions of the system that revealer is Christ, whose function is to teach rather than to atone; the crucifixion is often marginal or reinterpreted, and in some texts he does not suffer at all.
The consequences run all the way down. If the maker of the world is not the highest god, the scriptures that praise him as such are at best partial, and several of these teachers read the Hebrew Bible against its own grain, treating the serpent of Eden as a benefactor and the prohibition as an attempt to keep humanity ignorant. If matter is not the work of the highest god, the body has no ultimate claim, although what follows from that in practice is disputed, and the ancient charge that these groups were therefore libertine is among the least reliable things reported about them.