It was created by the Universal Machine inside Erath for those bored with the City. Unbeknownst to it, Antipathy had interposed itself in the gap between the City and the next universe, so the departing citizens were entering its timeship body instead.
Putting its consciousness inside one of the immigrants, he entered the city to take revenge on his mother for having given birth to him. Godfather Avatar destroyed himself and Antipathy with an annihilation bomb. The mindless timeship body continued to exist, and the Universal Machine decided to send more people there to escape the civil war that had broken out. Once all those who the Machine wanted to send had crossed over, it shut down the Gate. (PROSE: Of the City of the Saved...)