Gastly has no true form, due to its body being 95% poisonous gas. However, it consistently appears as a black, spherical Pokémon surrounded by a purple haze. It has a wide, pink mouth with two visible fangs. Though its eyes seem to extend past its round body, visible eyelids surround them. The toxic gas surrounding the main body can induce fainting and suffocation, and is capable of enshrouding an Indian Elephant within two seconds. Because it is mostly gas, Gastly will dwindle away when exposed to strong winds.
The anime suggests that Gastly has a playful personality, often frightening and playing pranks on people for fun. However, it has shown a willingness to preserve or pass on the memories of departed humans. It has further been shown changing form and creating illusions, and it is capable of phasing through solid objects and forming tangible hands out of its gasses. Gastly is often seen in old, dilapidated buildings, but can also be found in caves.
In the anime
Gastly in the anime
Gastly first appeared in The Ghost of Maiden's Peak. This Gastly had the ability to talk and shape-shift. He kept posing himself as the spirit of the woman which legend said turned to stone after 2,000 years of waiting for her love to return to her. As the ghost of the maiden, Gastly kept on toying with the minds of young men.
A pair of Gastly were among a group of Ghost Pokémon living in an abandoned mine in Fear Factor Phony. The Ghost Pokémon couldn't stand the partying Psychic Pokémon that lived in the nearby town, so they created a Haunter-like illusion to scare everyone away.
Gastly first appears in That Awful Arbok! under the ownership of Koga, possessing the corpses of dead Pokémon in Pokémon Tower. Red's Bulbasaur manages to defeat it by sucking up the surrounding air with its bulb, along with Gastly's gaseous form, and expelling it in a SolarBeam. In the process, the two also freed Red's rival Blue, who was under its ghostly possession.
Gastly is set to appear in the 3DS version as an enemy in Smash Run mode.
Pokémon Pinball: Three Gastly appear in a bonus stage accessible from the Red Table. Once the player knocks them out, they must then proceed to defeat the subsequent pair of Haunter and the Gengar that appears.
This Pokémon was unavailable prior to Generation I.
Gastly is largely composed of gaseous matter. When exposed to a strong wind, the gaseous body quickly dwindles away. Groups of this Pokémon cluster under the eaves of houses to escape the ravages of wind.
Despite all of them being part Poison type, Gastly together with its evolutionary family are not able to learn Poison-type moves by leveling up.
Gastly is tied with Venipede for the lowest HP of all Poison-type Pokémon.
Gastly seems to be based on general cartoonish ghosts and spirits, or maybe a will o' the wisp. It may also have been based on the Japanese youkai sōgen bi, the decapitated head of a monk whose head is doomed to wander the Earth, consumed by fire. Its Poison type is most likely a reference to poisonous or polluted gases.
Gastly may be a combination of gas and ghastly (dreadfully frightening or horrible).
Ghos is a shortened form of ghost and may also incorporate gas into its name.