holy smokes, it took a bit of digging to find them but you’re completely right! it basically outright says that the helmet turns folk nasty in The Golden Helmet (W OS 408-02), and even if Gladstone’s a bit of a pill in the Lost Charts of Columbus (D 94144) you couldn’t say that his behaviour is out of his usual character- and he clearly doesn’t care about owning everybody or making them do what he wants like the others. Maybe he does have an immunity to curses!
In which case, kind of makes me think my answer to this question-
might be that it’s a good thing that Scrooge has got a relative like Gladstone knocking about, just in case… ( though I think he’s been hoarding things like that for a looong time in the comics, so)