While not strictly "vulnerable" to gold, Superman is significantly weakened by kryptonite, a green radioactive mineral. In some comic book stories, kryptonite has been depicted as being able to absorb gold, making gold-plated kryptonite a powerful weapon against the Man of Steel.
However, this is not a consistent element of Superman's lore. He is primarily vulnerable to green kryptonite, and gold plating does not inherently make kryptonite more dangerous to him.
It's important to note that comic book continuity is often complex and inconsistent. So while this concept might exist in some stories, it's not a universal rule.