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The Gas type is a fanon type introduced in Pokémon Mighty & Nasty versions, started as a DLC and in-game available in later games. Gas type is actually based on gaseous molecules such as Gas itself, smoke, mist and anything that had no hard forms nor surfaces aside of dusts, flames, water, light and darkness. Gas type is used to balance Fire in term of offensive and defensive type effectiveness.
In offensive terms, Gas is strong against Fire, Water, Electric, as well as another fanon-type, Jungle. However, it is resisted by Light, Dark, Ghost and Poison. Defensively, it can resists Fire, Electric, Bug, Fighting and Psychic but is vulnerable to Dark, Light, Poison and either Robot or Cyber, whether of the two is available. The biggest threats are Dark and Light, while Poison is usable.
Gas is well balanced, and shares slowness and health with Water, but is more defensively built, while still good to hit hard.
While the total numbers of Gas-Type Pokémon is still pending, multiple official Pokémon like Weezing gained such Type. Pure Gas Pokémon is actually rare, as many Gas Pokémon are dual-typed.
Gas Pokémon have mixed amount of Egg Groups, and are based on creatures that is related to gaseous particles like smoke, and behavior is depended on the used particles; Pokémon with gas tend to be familiar with Poison, while Pokémon based on mist or vapor have familiar characteristics of Water, but some have more unique behaviors than others.
In Shadow Inferno's depiction of the Gas-Type, the type has actually been split into two sub-types which share many similarities with the parent type, these types are the Mist Type and Fume Type and effectively make up the two halves of the Gas-Type's primary demographics and share much of the same weaknesses and strengths as their parent type.