This was chosen not to air in the United States due to the heavy use of strobe lights which could trigger seizures.
When the episode was originally conceived in 2001, Jack Black was approached and agreed to do the voice of the Gnome, but by the time the episode entered production, Black's career had taken off and was too expensive for the show. Beatles member Ringo Starr was also approached but declined.
This episode was aired in other countries, expect for the United States, which no longer aired on television due to flashing seizures.
It was originally intended to be a series finale, but this was a season 5 episode, and it was never aired in the U.S. for unknown reasons.