since no one else is posting what gnosticism is actually about.
the root word of gnosticism is gnosis, which is a knowledge beyond intellectual knowledge. gnostics believed in most of the things that christ taught, but they believed that the way to salvation, and the way to move towards god is through a personal journey of inquiry and discovery instead of through christ as a intermediary, even though christ himself said "the kingdom of god is within you”
but of course you can't profit off that, or use it to control people, so it was systematically purged from civilization
this split is also eerily reminiscent of the difference between the historical buddhism talk by siddhartha gautama (theravada), which was presented as a personal journey towards enlightenment, and mahayana which came afterwards and became the main form of buddhism, where you can just put your faith in intermediate powers and you don't have to do the work of enlightenment yourself to get some of the benefits of it

anyways ,all the cosmology stuff about sophia, demiurge, the pleuroma, etc is all just window dressing on what is very simple. which is the idea that the path towards understanding the nature of reality and our place in it is a personal one, and when you reach the point that you have a "religious experience" its pretty unmistable. that's gnosis, a knowledge of something no one can give to you, you have to find it yourself.
if you want to learn more i'd start with valentinus and marcion of sinope