Gateway anime 

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.

Pretty much any anime on Cartoon network is a gateway anime.

Gateway anime by TabsTheGreat May 21, 2019

Gateway Anime 

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.

Two examples of Gateway Animes are:
-The Powerpuff Girls (who can technically be classified as chibis)
-The 2003-2006 version of Teen Titans

Gateway Anime by The Lovely France May 31, 2013