Alright, well, let me wade into this.
Oddly enough, I'm going to start by pointing to The Inhumans, the Marvel Comics property, whose character Gorgon debuted in 1965. Gorgon was described as having phenomenal strength and bull-like hooves, a combination that allowed him to stomp the ground and produce destructive seismic waves. Coincidentally, another Inhuman debuting earlier that year is Medusa. So the idea of "gorgon" referring to a bull-like creature and being distinct from Medusa was already in place before D&D launched.
This is of course not the first time this question has been raised, and while I have never found anything conclusive - which would pretty much have to be Gygax saying "so then I did this, and this is why" in an interview or column - I have followed the trail to the most likely culprit.
...which is this, The Historie of Foure-Footed Beastes:
The Historie of Foure-Footed Beastes is a book from 1607 detailing mythical and legendary beings as described by one Edward Topsell, who appears to have believed he was writing about real animals he'd simply never seen before. Here he speaks of the "Lybian Beast" or "Gorgon," a dreadful bull-like creature which feasts on poisonous herbs and so possesses a lethal breath.
Did Gygax read this book? We have no proof, though we have corroboration from another entry in The Historie which treats the lamia as a species of creature, half woman and half lion. Considering the mythological Lamia is an individual and a child-eating demoness by trade, the similarity of The Historie's version to the D&D interpretation cannot be overlooked. There's also the "Wilde Beast in the New found World called SU," a monkey-like critter reminiscent of another oddball and unique D&D creature.
Further corroboration comes from Jon Peterson's history Playing at the World: A History of Simulating Wars, People and Fantastic Adventures from Chess to Role-Playing Games, which also attributes the development of D&D's Gorgon to Topsell's work:
From Topsell, for example, Dungeons & Dragons inherits a gorgon that looks like a bull and petrifies with its breath, rather than the gorgons of the story of Perseus, of whom Medusa is the most famous. (Chapter 2.6, at p.154)
I've seen other references that use "African gorgon" to describe the catoblepas, which Topsell's book was referencing. Of course, the catoblepas is a separate creature in D&D — but then, it's more known for a lethal gaze than deadly breath. Thus, despite similar mythology informing both Topsell's "gorgon" and the D&D catoblepas, two distinctive monsters are produced.
What about the metallic hide? Well, for that, let's look at another mythical creature with which Gygax was likely familiar. The Colchis Bulls, or khalkotauroi, were mighty and terrible bronze bulls that could breathe fire. Overcoming them was a task with which Jason, of Argonauts fame, was charged. It may be that Gygax liked the myth but wanted to find a more accessible name than "khalkotauroi" and one less real-world-referencing than "Colchis Bulls," and inadvertently determined from various readings that Topsell's bull-monster with the killer breath and the other dangerous legendary bulls with ultra halitosis were references to the same creature. Thus was produced a thing with the best of both worlds - metallic hide, poisonous breath, and no unfortunate neck deficiency.
Needless to say this is all speculation, though it does look to sources that refer to the "gorgon" as a bovine creature.
