The party is given the power to decide between Heaven on Earth or Hell on Earth by either opening the Gates of Heaven and locking the Gates of Hell, or vice-versa. Heaven is described as a perfectly orderly universe where there is no pain and suffering, but there is also no pleasure and free will. Hell is described as being the exact opposite, a world where nothing is permanent and the heights of pain and pleasure and happiness and sorrow and free will go to the extremes.
Of course, the party can Take a Third Option, and lock both Gates (this is what the example party in one in the related short story does). This results in the God Machine and its Angels as well as Demons being permanently cut off from the world, and humanity is free to forge its own destiny. Basically, the World of Darkness becomes the normal world, with no more supernatural influence.
What gate?