You're not the first to postulate this, and it has a strong possibility of being correct. Still, we don't have enough canon information to say one way or the other.
Gigi is not the real Guildmaster. He's a Mammet that was owned by the real Guildmaster, and is now owned by Serendipity, the acting Guildmaster. The real Guildmaster never makes an appearance, in my recollection.
And while Gigi lambastes your work at every opportunity, it's made pretty clear that he's literally unpleasable. You could present him with a masterwork par excellence, and he'd still spit on it. Serendipity, on the other hand, is blown away by everything you do, just like other Guildmasters.
Both Gigi and Serendipity are left behind past level 50. From 50 to 60, your new contact is Marcel, who also cannot stop gushing over everything you do. Moving on to 60 to 70, you become a Goldsmithing instructor to a class of students; all of them take it as a given that everything you touch turns to gold, and can only hope to one day manage a fraction of your genius.
So, no. Goldsmithing is not an exception. XD