The Gate Rune was safeguarded by the Clan of the Gate and when that clan was destroyed, the rune was used by Windy and Leknaat to escape the massacre. Leknaat considered the Gate Rune to hold terrible power.
The rune is, in fact, two runes, the "front" and "back" (also called the "entrance" and "exit") runes. The rune was split between the two clan sisters with Leknaat bearing the "back" rune and Windy the "front." The "front" rune was said to be able to summon from other worlds, while the "back" rune was able to reverse such summons. Their powers are considered to be connected.
Combined with the power of the 108 Stars, the Gate Rune was used to bring Gremio back from the dead, although Leknaat stated this to be a unique event. The Rune of the Gate could also return monsters to the nether world although, like all runes, it is nullified completely by the Sovereign Rune.
Windy attempted to use her rune to achieve political control of the Scarlet Moon Empire to further her hunt for the Rune of Life and Death. Towards the end of the Gate Rune War, she summoned thousands of monsters to bolster the Empire's forces before the final battle but had her creatures returned by the combined power of Leknaat's "back" rune and Joshua Levenheit's Dragon Rune.
The whereabouts of Windy's "front" half of the rune is unknown following the Gate Rune War, along with Windy herself. The "back" half remained in the possession of Leknaat.
- Genso Suikoden Kiwami Encyclopedia, pages 106, 225