Gastly appears to be nothing more than a black ball of ghostly matter or gas that is surrounded by a purple glow of smog. Gastly has big, white eyes and a mouth with a pair of fangs in it. Gastly's face is similar to Cloyster's, but the two Pokémon aren't related.
Due to having a body made of gas, Gastly can slip into any place it wants. However, Gastly's body will dwindle away when exposed to a strong wind but it can probably regenerate itself later. A Gastly is capable of toppling an Indian Elephant within two seconds by enveloping it in poisonous gas.
There have been claims that on rare occasions, a Gastly can gain the ability to speak like a human and transform itself. However, it apparently can communicate with telepathy.
Gastly can learn and use the elemental punches, despite appearing to have no hands. Though it can create hands from its gas.
As a Ghost-type Pokémon, Gastly tend to be pranksters that go scaring people just for the fun of it. They are also proficient at sneaking up on unsuspecting prey and putting it to sleep or poisoning it through their skin. As Gastly's body is blown away by wind, groups of them gather under the eaves of houses to prevent being blown away.
Gastly live in old, dilapidated buildings and can also be found at cemeteries. It is a common sight in Kanto and Johto.
As the victims of Gastly's gas enveloping technique have been called prey, it can be presumed that it derives energy from them. It may also lick its prey.
This Pokémon was unavailable prior to Generation I.