This article is brief.
NNGC (probably standing for New or Next Nintendo GameCube) was an internal codename for the Wii project, used before the "Revolution" (RVL) name was decided on. It is referenced in various places, including the names of the NDEV USB drivers and game files of The Legend of Zelda: Twilight Princess. It has been theorized that the NDEV's full name is "NNGC Dev".
According to SDK header files, the NNGC name fell out of use around July 2005, and files named "NNGC" were changed to Revolution or RVL.