Reading Belial's Brood and it says, in the standard I will neither confirm nor deny the truth of this, that Belial is the right hand of the Demiurge. Now, I feel like something else has been called the "Demiurge", probably The God Machine, but maybe in relation to Demon or Promethean (which still might make it the God Machine). Can anyone clarify what else (and where) has been referred to as the Demiurge?
You are right that Demiurge is a term already in use by another game line. The Lexicon of Promethean: the Created defines a Demiurge as a mortal human who wields Divine Fire to create a Promethean, who then becomes the Progenitor of a new Lineage of the Created. (p. 26) Saturnine Night Chapter 1: "Demiurges in the Modern Age" goes into extensive detail on how and why they arise.
I'm surprised no one brought up Mage yet. Mage borrows much of its cosmology from Gnosticism, and there are strong correlations between the Exarchs and the Demiurge. Both are said to be hubristic jailers who keep the Supernal souls of humanity trapped in the Fallen World. The Gnostic Demiurge is explicitly referenced in Tome of the Watchtowers as a symbol or metaphor used by the Mastigos and Obrimos Paths: Mastigos for its connection to Arabic & Islamic folklore (p. 55 & 57), and Obrimos because it often draws upon religious imagery, Gnostic Christianity being a favorite (p. 113). Some Obrimos Banishers reject the divine Presence they felt in the Aether, seeing it instead as a deceitful demiurge. (Banishers p. 37) The Gaolers of Ialdabaoth were an Abyssal-hunting Legacy survived only by their Codex; their Enfeeblement (Life ••••) rote "Humble the False Leviathan" involves "reciting Gnostic verses abjuring creatures of the demiurge" to weaken Abyssals that take physical form. (Grimoire of Grimoires p. 86) Mystagogues point out that the Platonic Demiurge differs from the Gnostic one, and its well-meaning attempts at fashioning a more perfect world are compared with the Mysterium's view of a living cosmos that changes itself. (The Mysterium p. 50)
Though the Demiurge doesn't seem to be explicitly mentioned in any of the books so far, Demon's subtitle is "a Storytelling Game of Techgnostic Espionage" which is certainly a reference to Erik Davis's book, Techgnosis, (Demon p. 25 even says so) but may also hint at a Gnostic inspiration for the God-Machine or its creations.
Hunter brings up demiurges, but almost always in the context of Prometheans. Mortal Remains Chapter 5: "To Hell and Back" describes the current and former agents of "the demiurge, a machine that has made Earth part of its mechanisms." (p. 120) The term is used throughout the chapter to refer to the God-Machine; Hunters seem to favor the term, perhaps because it conflicts less with the beliefs held by many hunters or their allies.
The term Demiurge comes from Gnosticism. The idea is that God (sometimes called the Monad) created the universe, but has largely been hands-off since them, and in particular, it is not responsible for creating our world. According to Gnostics, the world was created by the Demiurge: a lesser god, created by the Monad, who has gone insane and thinks that it is the only God. The Demiurge, in its insanity, has become malevolent, yet it is also pitiable for its blindness.
From that overly-simplified description, it becomes easy to draw parallels between the Gnostic Demiurge and the God-Machine: a malevolent creator whose very nature suggests that it too must have had a creator once, yet it remains unaware of this. But I don't think it's safe to assume that the Demiurge and the God-Machine are the same being. It's too comforting a concept for the World of Darkness: if the God-Machine is truly playing the role of the Demiurge, then there must be a Monad somewhere, and that kind of hope is not something this setting is known for.