Any film, video, or animation, that has had a history of causing normies to take an interest in anime, regardless of where or when it was made.

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An anime-like show found on Cartoon Network or Boomerang that, though the art style resembles that of anime, cannot be qualified as such due to its non-Japanese origins. However, these shows have the capability of introducing the viewer to the anime art style and general fandom, hence the name "Gateway."

Pokemon and shows of the same type usually found on Cartoon Network also qualify as Gateway Anime, since they have been Americanized to the point of ridiculousness.

In short, Gateway Anime is anything even remotely anime-like that pops up on Cartoon Network, usually early in the morning or late at night.