Gjallahorn will never be sold by XUR until HOUSE OF WOLVES. Even then they may not sell it because they aren't coming out with another exotic launcher. The HOW DLC has 2 of each weapons and 2 armours per class that will be new and exotic along with around 20-30 legendary guns. The Gjallahorn is just going to be upgraded to 364 damage like all exotics and legendaries will be in the range of 340-350. Honestly grind for the GHORN by ROC strikes and Crucible. Best bet. I got lucky and got my GHORN from soloing the Daily (The Last Array).