It's probably a G4. When the gamecube was on a TV show last year, the reviewer was trying to explain in non techy terms what powered it and he said it was basically the same chip that the Apple Macintosh uses. It was on the Big Breakfast (a british TV show).
Games consoles are often claimed to be a certain Bit when they're not, they just pick something fast and use that in the marketing. A 64bit GPU would technically be enough for them to market their consoles as a 64bit console, the G4 having a 128bit vector unit could be what they're refering to. Then again it could be the chip on Ati radeon that powers the game cube's graphics.