If you want to understand what the gnostics were talking about you need to know the basic cosmology widely accepted at the time. Cicero summarizes it in his Dream of Scipio passage at the end of his Republic:
See! the universe is linked together in nine circles or rather spheres; one of which is that of the heavens, the outermost of all, which embraces all the other spheres, the supreme deity, which keeps in and holds together all the others; and to this are attached those everlasting orbits of the stars. Beneath this there lie seven, which turn backwards with a counter revolution to the heavens; and of these spheres that star holds one, which men on earth call Saturn's star.
Next is that bright radiance, rich in hope and healing for the sons of men, which is called Jove's star; then one fiery red and dreaded by the world, which you call Mars; next lower down the sun holds nearly the middle region, the leader, chief and ruler of the other lights, the mind and ordering spirit of the universe, of such magnitude that he illumines the whole and fills it with his light. With him Venus and Mercury keep pace as satellites in their successive spheres; and in the lowest zone of all the moon revolves lighted up by the rays of the sun.
Now below these there is nothing more but what is mortal and transient except those souls which the bounty of the Gods has given to the sons of men; above the moon all is eternal. As for the earth, the ninth and central globe, it does not move but is the lowest point, and towards it all heavy bodies tend by their own gravity."
So Saturn is not just what we call the planet Saturn but also the penultimate heavenly sphere in which this planet is embedded, which contains everything below it. Earth is the lowest, or centermost, sphere.
From the perspective of modern astronomy this entire model is untenable. To many ancients, including the late antique writers called “gnostics,” though, this was, with variations, the basic map of the cosmos. The stars/ planets were generally held to influence life on earth- for some thinkers this was a vague influence while for others it was the irresistible mechanism of fate. Stoics and neoplatonists believed that the decrees of fate/providence were necessarily good- gnostics and some hermetists would strongly disagree and view the planetary spheres and their ruling gods as oppressors and prison guards. When Paul calls Satan “archon of the power of the air” and talks about “the powers of wickedness in the high places” (note that Paul does not place Satan in an underworld but in the heavens) he is also expressing some version of this idea.
The figure of the Demiurge was introduced in Plato’s key dialogue Timaeus. If you haven’t read that yet drop everything else and read it because that is the foundation of the whole concept that later thinkers are responding to. You should also check out later developments of the concept such as found in the Enneads of Plotinus. We live in a time where a vast quantity of these ancient texts are readily available in translation, often for free, so take full advantage of that. Simply knowing these texts will put you in a much better position to evaluate anyone’s claims to carry on “the wisdom of the ancients.”
Plato’s original Demiurge is a good/ wise figure who fashions the world on the basis of the Forms. The reason for the imperfection of the world is not ill will or stupidity in his part but the inherent unreliability of matter. The exact place of this demiurge in the cosmos and the problem of matter is developed and debated extensively in later writings and it is a good idea to go over the full range of that debate before settling on the solution of a particular current.
