Hey remvik,
Prior to the release of Destiny, Bungie released redeemable codes. One of these codes contains an entry for the Gjallarhorn. If you have the grimoire card for it, but no weapon, that would explain why.
You can find out more here: https://www.bungie.net/7_I-have-a-Grimoire-card-unlocked-for-Gjallarhorn-bu/en/Help/Article/12652
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And that answers that.. during the beta a friend of mine fed my bungie account with all the codes he could get his hands on. I never bothered to follow up on it. Such a play on my emotions.. gjallahorn, why u no love me? :'(