I'm in a fight mode, so sorry in advance. But my point is, you shouldn't stress out about a single word from a different language and claiming it as being the only way to pronounce it. Try the following words: Thor, Odin, Ragnarök, Loke, Schwarzenegger. No, you didn't pronounce them correctly in your head, and you never will. It doesn't matter. But don't try to be smart if you can't back up your case. Nordic mythology is based on the Edda, a collection of fairytales about the Nordic gods and their adventures. Teh word Gjallarhorn was spelled in a different way a long time ago as all other ancient words from the bible, for example. So how it is pronounced in today's writing, it doesn't matter. I'm swedish btw. I pronounce it Yallahoorrn. My german friends pronounce it with a G mostly. Rant over.