The Gungnir (グングニル, also officially translated Gunguneel in Fire Emblem Treasure) is one of the holy weapons in Fire Emblem: Genealogy of the Holy War, and was the weapon of the Crusader Dain.
The holy spear of the Thracian dragonkings, it is similar to the Gáe Bolg, but it gives a bonus to Speed in place of Skill.
It is used by Travant against Quan, who at this point has the Gáe Bolg, and Ethlyn when both appear in Chapter 5 on your side (but not under player control). It is later used against you by Areone during Chapter 9 and in the Final Chapter, but if Altena talks to him in the Final Chapter, Areone will come to your side (but not under player control, instead, he and his Dragon Knights will take formation around Altena).
Only units with major Dain holy blood can wield the Gungnir, and since players do not have units with major Dain blood, the Gungnir is not even obtainable in FE4.