You need special mechanics for Gustbran smiths in the same way that you need special mechanics for non Gustbran smiths, which is to say, not at all. Making a valuable and highly effective mundane sword is the same as making a magic sword, or a good cart or gold necklace. HeroQuest is not about swords, it's about stories and people. What effect does the sword have on the story? A gift? Use the smith's ability to persuade the recipient. To help a fighter? Use the ability as an augment. A bane sword for a specific foe? A plot augment, perhaps.
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