Gungnir is one of the Four Treasures, and is also the greatest and most powerful of them. Gungnir maintains equilibrium in Asgard, and its wielder is the ruler of the Aesir. It is an important story element in Valkyrie Profile 2: Silmeria.
Gungnir grants it's wielder phenomenal power, as exhibited by Odin and Lezard Valeth. The enormity of it's strength is never truly revealed, but it's wielders are all able to perform extremely powerful attacks in battle whilst in possession of it.
Valkyrie Profile 2: SilmeriaEdit
Warning: Spoilers follow! Odin wields Gungnir in Valkyrie Profile 2, and does so for most of the game. In Chapter 5, he takes his spear with him to the top of Yggdrasil to confront Alicia and Rufus. He engages in combat with them, and finds Lezard Valeth. However, he is disarmed by the mage, and Gungnir is stolen from him. Lezard uses the Treasure to create a new realm, and using it's power attains godhood. He is defeated however, by Valkyrie, and Gungnir is taken by Rufus to rule the Aesir. In the Seraphic Gate, however, it can be obtained by the party, and is usable by Hrist.
Spoilers end here
Gungnir is a powerful spear wielded by Odin in Norse Mythology, and much of the lore surrounding it is replicated in Valkyrie Profile.