Earlier quoted context omitted.
Well, they're painted. And you'd think that would still mean rust where they were scratched, cut, etc. But OP's magnets stuck to a drip edge, which I wouldn't think would be made out of plain steel either. I'm assuming my gutters aren't ferromagnetic, but I could be pleasantly surprised!
On US homes, drips are typically galvanized steel because they are a commoditized roofing product. Gutters are usually aluminum because gutters and downspouts are a specialized trade and those businesses fabricate components on-site from aluminum roll stock to correctly fit the building.
The gutters have seams and weren't formed on site, but are also non-ferromagnetic. So much for the hope of quick and easy mounting.
If I had to guess - while galvanized steel wouldn't rust on its own, it's avoided due to the possibility of being in contact with other metals that would cause it to still rust. Roofing nails are galvanized, but they're supposed to be dry under the shingles.