User blog comment:Silicon Soldier/Xûr's Inventory: July 10, 2015 - July 12, 2015/@comment-26037220-20150711023525/@comment-25916964-20150711112142

If your waiting for a particular item, it might be worth checking out Bungie's new Destiny Armory. It's got a list of every possible inventory item (the Gjallarhorn is on there, so it should appear... eventually...) At least this way you'll know if what your after will ever show up (providing the RNG gods are feeling generous)

Possible Xur Inventory @ Bungie.net