This would refer to armor that was primarily metal. As in, you would look at it and say "that armor is made of metal." As opposed to "That is cloth armor" or "that is leather armor." This is supported by a quotation from the Sage Advice column, where it is talking directly about the Druidic Taboo against wearing metal armor.
The idea is that druids prefer to be protected by animal skins, wood, and other natural materials that aren’t the worked metal that is associated with civilization. Druids don’t lack the ability to wear metal armor. They choose not to wear it.
[...]
A druid typically wears leather, studded leather, or hide armor, and if a druid comes across scale mail made of a material other than metal, the druid might wear it.
http://dnd.wizards.com/articles/features/rules-answers-march-2016
Here, we can see that the metal fittings that would normally be added to studded leather do not count as 'Metal Armor.' And, strictly speaking, you can make Leather Armor with fittings other than metal rivets. I am actually a hobbyist Leatherworker and I do know how to make armor. Ideally, you use metal rivets to hold the pieces together...but it's entirely possible to use stitching, bone, or other hard materials in place of the metal.
For a simple test, just read the description of the armor in the PHB. Every description emphasizes the primary material that the armor is made from. For example...
Studded Leather. Made from tough but flexible leather,
and
Chain Shirt. Made of interlocking metal rings[...]
and as a final interesting point...
Ring Mail. This armor is leather armor with heavy rings sewn into it [...]
In short, while most of this will be DM-fiat, and it's quite possible to have Dragonscale Scale Mail or a Breastplate made from the carapace of some insectoid creature...as a general rule of thumb, if the description of the armor in the PHB explicitly mentions metal, then you can likely assume that piece of armor counts as 'metal' for the purposes of Druids, Spells, etc.
No Corrosion
A Gorgon is a creature with Natural Armor and is not an object. Rust monsters only corrode objects. Being Natural Armor, it is not an object on top of their actual body, but a part of their body.
Organic Metal?
It's Horrific Structure is also described organically (emphasis mine):
A gorgon’s iron plates range from steely black to gleaming silver, but this natural armor in no way hinders its movement or mobility. The oils of its body lubricate the armor. A sick or inactive gorgon gathers rust like fungus or mange. When a rusty gorgon moves, its plates squeal as they rub together.
Even if you allowed for the Rust monster to affect the Gorgon's Natural Armor, it wouldn't do much besides reduce their ability to be sneaky.
Best Answer
TL;DR: it prefers to target ferrous metal
This is likely a hold-over from earlier editions, which had this text in the 2e Monster Manual entry for Rust Monster:
In 3rd edition, the Monster Manual has this text:
(my emphasis)
Thus, the action describes a preferred target for the creature, being ferrous metals. It could well be that the ability might also corrode a non-ferrous metal .. but with 5e's "what the rule text says but nothing more" general principle, then it actually wouldn't, RAW.
So, generally, the statement "But seeing as bronze is not ferrous, it cannot be targeted" is erroneous, as it should really read "it won't be targeted".
There are also two "Ecology of the Rust Monster" articles in Dragon Magazine #88 and #346 that go more into the lore. Take what you read there with a pinch of salt, as some text contradicts earlier versions.