Item description:

This mod makes the appearance of Oblivion gates throughout the world based on time, rather than chance. There will also be random Dremora Raiders spawning from the gates. The more gates that remain open, the more likely things will spawn. Close the gates or be overrun with the hordes of oblivion!

There's no need to start a new game to use this mod either. Everything is time controlled, and will be updated as soon as you load the module.