In the first edition of the New World of Darkness, the presence of the God Machine was subtly hinted at in the other core books.
Danse Macabre, a sourcebook for Vampire: The Requiem, details a mini-Covenant called the Holy Engineers, who receive transmissions from the God Machine that they strive to interpret. The intended effect is "oracles in reverse" — they'll receive answers and missions to undertake with the goal of figuring out the question or inciting incidents.
Imperial Mysteries, the archmastery book for Mage: The Awakening, says this:
It might also be possible to secretly change the world into an occult engine for a Fallen god: a Mystery Play that returns it to power. The ententes believe at least one such "God Machine" already exists.
The following contains spoilers for "These Mortal Engines" in the Promethean: the Created sourcebook, Saturnine Night.
The God Machine, or an aspect of it, makes an appearance at the climax of this adventure, "offering" the player characters a chance to become human. The choice to activate or destroy the machine is crucial to the final act.
In Demon: the Descent and the second edition New World of Darkness games, the God Machine takes on a much greater significance.
In Demon: the Descent, the God Machine and its angels are the primary antagonist group for the Unchained, who themselves once served the God Machine as agents before Falling and going rogue in pursuit of their own personal Paradises or Hells.
In The God Machine Chronicle, which is an interim book standing in for the forthcoming World of Darkness Second Edition, the God Machine serves as the driver for an entire set of adventures involving the secret demiurge of the world.
In Vampire: The Requiem Second Edition, the God Machine is given as one possible cause for the anticovenant known as VII.