How to get the Year 3 Gjallarhorn and infuse it with Iron Gjallarhorn
The new Iron Gjallarhorn is a Pre-order bonus for Destiny: Rise of Iron and is a reskin of the regular vanilla Gjallarhorn. When you first enter the new Rise of Iron, you can pick the Iron Gjallarhorn up from the postmaster, but there is a downside to your shiny new rocket launcher.
Before you can use it, you have to find the year 3 Gjallarhorn first and this is done through the new story quests in Rise of Iron. Make your way through the story to obtain the rocket launcher and you are good to go. Once you did this you can infuse the year 3 version with the Iron Skin from the postmaster to give it the new black paintjob. That is all.
But again, this is only possible if you pre-ordered the game and got the Iron Gjallarhorn bonus. You can still get the regular one without it though.