The Gates of Eternity

History
It's origins unknown, The Gates of Eternity (ザーゲイト　アヴイターニチ） are a mysterious plot element to the SSG franchise which appears in all installments of the franchise. The Gates are often thought to be able to use it's power to commit impossible acts such as grant immense power and even bring back people from the dead.  Unless used by an extremely powerful being, the Gates can only be used once per millennia, and those of the most impure hearts are Punished for being in it's presence by dragging them into the doors and closing them shut, making those trapped in the Eternal Darkness for all eternity.

Appearances
The Gates appear in all the installments, most notably in the end of the original series as well as the end of S4TURN (the 2004 remake). Also it is one of the most important plot element from the third film of the S4TURN remake films.

Fan Speculations
Many fans theorize the the Gates of Eternity are actually a stand. Some believe that it is The Belladonna of Sadness' stand while others believe it is 『̶̵̨̨́O̴͘N̵̶͝E̸̡̧ ̡͞Ḿ̀͠O̸̡͟͠Ŕ̴͝͝Ȩ͘͢͢͏ ̀͜͝T͜͟͏I̢̛͜M͠͏̡͜E͏͢'s』̛͠  second stand, but the author has never confirmed whether this interpretation is true or not.