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Shortly after the release of "The Castle", the thirty-third episode of Season 2 of The Amazing World of Gumball, a short NSFW animated parody of the episode called "The Shyrocket" was uploaded to YouTube.
The parody animation starts off with a well-made, authentic looking title card from the show. The card reads "The Shyrocket". After the title card appears, it cuts to Gumball sitting on his family's recliner with Harold Wilson. In the original episode, there's a running gag where Harold pokes everyone, including Gumball and his family. In the NSFW parody, soundclips are taken from the original episode, and Harold is poking Gumball's penis repeatedly. Gumball is visibly disturbed. Currently, there is no recollection as to how the parody ended, but it presumably ended with credits to the original animator and perhaps to Cartoon Network/The Amazing World of Gumball's respective owners.
(EDIT) Found a GIF of the animation in a forum