Valentinianism might be a interesting place to start. The cosmology is pretty complex but also interesting if you have the mind to bother with it. Not saying that I personally subscribe to it, just that it gives you a good idea how fleshed out one of the Christian Gnostic schools can be.

What's also interesting, The Lord of the Rings lore includes a creation tale in the Silmarillion. J. R. R. Tolkien was a Catholic but he cooked up a really interesting cosmology and creation story that is part biblical retelling, but with a sort of Gnostic flare. The idea of sound or vibration and different harmonies sung by the Ainur to shape the universe of creation. That in itself isn't really a gnostic idea but it's pretty cool none the less.