First EditionHere, gorgons are described as bull-like monsters covered in metal scales. The illustration confirms this, with polygonal scales covering the back and flanks, and more linear plates over the chest and the hind surface of the forelimbs. The creature does, however, have dark shaggy hair over the top of the head, and the horns are wide, curving upwards. The exact size of a gorgon is unclear, although it's physically much tougher than a regular bull, and may therefore be intended to be larger. They are also slower than regular cattle, presumably because of the weight of their armoured skin. Unlike cattle, gorgons do not seem to be herd animals, but they clearly have an animal-level intelligence, and hence, no real alignment.
Second EditionGorgons are confirmed to be "bull-like beasts" resembling "monstrous bulls" with "bull-nostrils". Which seems fairly unambiguous. We now know that the metal scales are black in colour, and they have red eyes. They have lost the hair on their head, although we can now also see the tail, which turns out to have a tuft of hair at the tip. Even the horns seem to be scaled, and, while the text says otherwise, they look to have short tusks in the lower jaw. We are told that they are 8 feet (240 cm) tall, but they can apparently walk on their hind legs, so this could either refer to a bipedal height (in which case, they're actually quite small, for bulls) or to a shoulder height (in which case, they are comparable to Indian elephants). Further information is given on their diet and habits.
Third EditionThe most obvious physical difference is a large ridge running down the back, with a row of peaked scales. The other scales seem more regularly arranged, and the silver-coloured horns sweep downwards and forward. The eyes seem to be dead black, and the teeth are flattened, and (unlike those of cattle) found in both jaws. The lower jaw comes to a sharp point at the tip. They explicitly have night vision, and can even "see" in the complete absence of light, as well as having an acute sense of smell. The size of the animal is more explicitly stated; larger than regular cattle, it's about the size and weight of a rhinoceros. And apparently they do now, at least some of the time, live in small herds.
Fifth EditionThe ridge on the back is smaller this time, but still clearly present, and with more jagged scales. The scales on the head and body takes the form of linear bands, rather than polygonal plates, and the horns curve dramatically forward and upward. The eyes are now silver, and the tail entirely hairless. Both black and silver-coloured gorgons seem to exist, reconciling the colour schemes of 2E and 3E, and the scales are specifically stated to be iron-based. For the first time, it seems to move quite swiftly.
So we have an animal that is generally bull-like in form, but larger than regular cattle, which is either hairless or nearly so, and has a body covered in metal plates - although the exact form of the plates is notably different in 5E than in earlier versions. The shape of the horns probably varies between individuals, as it does between breeds of domestic cattle. Although the scales apparently weigh less than one might expect, they are usually sufficient to slow it down a little, compared with regular cattle.
Given their bovine anatomy, it seems hard to believe, as 2E claims, that they can walk bipedally. There's no hint of this in other editions, although the picture in 1E does at least appear to show an animal rearing up on its hind limbs, probably in order to trample an opponent.
That the scales are metallic implies a somewhat alien biochemistry, and, presumably, a high mineral intake. We are told in 5E that a gorgon's skin secretes an oil, likely from modified sebaceous glands, that lubricates the plates and protects them from rusting. It also seems plausible to suppose that a gorgon would also be able to use solid rock surfaces to scrape rust from its body, regrowing the plates from metabolic iron reserves. Physiologically, they might be depositing metallic iron into a sheet of cortical bone formed in the same way as an armadillo's armour; it's not terribly easy to see how this would work at a molecular level, but... hey, magic.
The natural habitat of gorgons is somewhat ambiguous. 3E states it to be open temperate plains, perhaps suggesting something like that of plains bison, but the flavour text in other editions implies a more forested environment, as would be typical of most other bovine species. A significant difference between gorgons and real bovines, however, is their primarily carnivorous diet, said to consist largely of deer (a woodland animal) in 2E.
2E also states, however, that gorgons let out a scream of rage once they sight potential prey. This sort of thing just doesn't happen in the real world; predators want to sneak up on their prey, not alerting them until it's too late for them to flee. It may look dramatic in action films, but, in reality, roars and the like are signs of territoriality, of trying to intimidate and drive off a perceived threat, rather than hunting for supper. With their large size and relatively slow speed, a forested environment, or at least one with significant cover, seems a better bet than open grasslands for predatory gorgons. Given their night vision and ability to track by scent, they may also be nocturnal, which might help.
Since we're repeatedly told that they look like bulls, rather than "cattle", it's reasonable to assume that there's relatively little sexual dimorphism amongst gorgons. That is, the females look much like the males without the sort of close examination that most people would be reluctant to undertake while the animal is still alive. It makes little sense for them to be pack hunters, and most editions agree that they live in small groups. They seem to be polygynous, with a male dominating up to three females and their calves - since this would require them to compete for mates, the males would likely be larger than females, even if they have a similar build and body-form in other respects.
It seems plausible that the calves are born without the metallic scales, given the moist environment of the womb. They probably develop rapidly, however, perhaps due to an exceptionally high iron content in the milk. Pregnancy in cows lasts nine months; it may be a few weeks longer in gorgons, because of their larger size, but likely not by much. 2E states that gorgons take a couple of years to reach sexual maturity, after which the males leave the group to challenge an older individual for breeding rights; this also fits with cattle, suitably adapted for a non-herd based social structure.
The signature attack of gorgons is their ability to exhale a cloud of greenish gas or vapour that turns living flesh to stone. This seems to work by skin contact, rather than inhalation, and does so rather more slowly in 5E than in earlier editions. The volume of gas produced is about 6300 cubic feet (180 m3), and whatever gland produces the stuff can hold enough for four or five blasts at a time, which may suggest that it's stored as a liquid and sprayed out as a fine mist. It's exhaled through the nostrils, so the gland is probably attached to the trachea, the vapour being expelled with a blast of de-oxygenated air from the lungs. This is because, since it doesn't hang about once exhaled, it presumably loses its potency rapidly on exposure to oxygen.
But the real question, of course, is why a gorgon would do this. On the face of it, it appears to be a defensive attack, and gorgons have little to fear on that front. Similarly, they are, naturally enough, immune to their own vapour, so they can't use it when competing with one another for mates or territory (not that they'd want to, since it would rather thin their population). Instead, it seems to be a means of obtaining nutrition, with gorgons actually feeding on the statues that they produce.
Which explains why they have blunt teeth, rather than sharp canines.
Given that this is magic, we can ignore any questions about conservation of energy that it might invoke, and instead simply wonder why they bother. After all, it can't simply be that they eat rock, or they'd just munch on the landscape without putting themselves at risk. It might be that their saliva or stomach acids turns the rock back into flesh, and that they're perfectly capable of eating regular meat, simply using the vapour as a means of rapidly immobilising prey. Most editions, however, seem to agree that petrifaction is their preferred form of attack, which would seem wasteful if they have to turn it back into meat again to digest it.
More likely then, the particular sort of rock than they turn their victims into gets them some trace nutrient - probably iron - that they can't acquire in sufficient quantities by other means. Their prey are being turned into some sort of haematite. This, it should be noted, wouldn't rule out the possibility of them also eating regular meat from time to time, so long as they can supplement it with the right kind of stone.
[Picture by LadyofHats, available under Creative Commons CC0 1.0 Universal Public Domain Dedication.]