The Gate Rune is one of the 27 True Runes. It has the power to open portals to other worlds and dimensions. The Rune of the Gate is in fact two runes, the “front” and “back” runes, also called the “entrance” and “exit” runes. Windy carries the “front” and Leknaat the “back”. Their powers are connected. The Pale Gate Rune is presumably descended from it. The Rune can also be used to summon powerful beings and monsters from parallel dimensions and thus was protected by the Gate Rune Clan to avoid it from falling into the wrong hands.
However, in the year SY 70, the Holy Kingdom of Harmonia raided the Gate Rune Clan in an attempt to seize the Gate Rune. Clan members Windy and Leknaat took the Gate Rune and split it into two, taking each piece with them to escape from Harmonian forces. Since then, this Rune has been split (into the Front Gate Rune and Back Gate Rune), and remains that way at present.
The back gate rune exhibited the power to open a dimension where a person is at the point of death and bring that person back to the present but that is only possible once. Windy brought monsters from the nether worlds to the present to aid Yuber and Kanaan. The powers of the back gate became weakened after using so much power to save Gremio thus, unable to return the monsters from where they came from.