I guess the logic is that iron can't be fully resmelted because it's a pure metal and not an alloy, and that would also apply to gold. Although even if you have a gold farm, it's much better to use the gold for more gearboxes.
edit: If you think about it, it isn't really realistic that you can get full ingots from melting down alloys but the same process makes you lose a portion of pure metals, that's sort of backwards. So it's just one of those things that is the way it is because the game would be unbalanced otherwise.
Edit 2: My point was the realism doesn't actually matter. Since the logic is fictional, it can be anything. If only one metal was nerfed then there would be a different, more arbitrary explanation for why that metal is different from the others. But gold gear is so weak and unimportant that it wasn't even worth sparing it the nerf.