Hey there, By completing the Quest you only receive the Gjallarhorn, the regular one. The Iron Gjallarhorn was a pre order bonus and is only available to the Players who pre ordered Destiny: Rise of Iron. If you have pre ordered and have obtained that weapon on another character, then please check the Exotic Weapon Collection Kiosk, found in the Tower in the Hall of Guardians (where the Vanguard Mentors stand), on the left wall when you look towards the exit of the room. You should be able to obtain another Iron Gjallarhorn from there, but only if you pre ordered.