I've been reading about Gnosticism and I'm confused. Do Gnostics believe in Hell or do they believe that Hell is on this earth? Do they believe that it's temporary or permanent?
This is one of the areas where there is a unity of thought in the Gnostic tradition. There is no literal hell in the Gnostic tradition. It is a state that exists for people here. No state of being is permanent here. Hell is a state of ignorance and suffering from being subject to the forces and powers of the world. Ancient texts use metaphors of being a slave, being asleep in a nightmare, of being drunk, and even of being dead. They saw that one could be redeemed from that state to some extent while in the world. That is why it isn't accurate to say that being in the world is being in hell. One of the sacraments/mysteries listed in the Gospel of Philip is the Apolytrosis, a word meaning to be redeemed, to be bought out of slavery. There are also texts that speak of the resurrection as something we experience here and now.
It is a good question as to whether any of the early Christians believed in hell. Certainly, they did not believe in anything similar to the concept as it exists today. It had a long development in Christian culture both in art and literature, and in preaching and theology.