Yeah, definitely it gets a bad rap because there are a few really bad actors who call themselves Gnostics. Most of us are reasonable people.
The general story goes something like this: There is God, most high, and a number of beings called "Aeons" emanated from him ("created" is a little wrong, because that would imply effort on God's half). There's a bunch of them that go create their own little Aeons, and the details of this are fuzzy and irrelevant, and different people disagree on it but are OK disagreeing on it.
One of these Aeons is Sophia. She wants to be like God and create an Aeon on her own, without her male partner Aeon. This goes real bad, she loses a fragment of her divinity, and it's embedded in this creature called the "Demiurge," or "Yaldabaoth." She panics, hides the Demiurge away in this tiny veil, and the Demiurge has no fucking clue what's going on. He creates the Earth we're all on right now, and a bunch of beings called "Archons."
Sophia realizes she needs to get her fragment of divinity back, and tricks Yaldabaoth into pouring that divinity into the Earth. Humans end up getting huge piles of this, and the goal of Humanity is to return the tiny fragment of divinity - often called a "spark" - within us back to Sophia.