The problem is if they nerf Gjallahorn then it won't be as good of a gun. Honestly use hunger if crota or any other 331 tracking rocket launcher and you will be fine for crota or pretty much anything else. I would like to see Xur sell Gjallahorn but without bungie nerfing it. There is no point in having one of its not going to be the most powerful gun in the game. So just be patient and keep trying you will get one eventually.