Nothing different. NSMB was so revolutionary because it brought the new and the old generations together to play a remixed version of the old SM games on a Handheld port.
Nintendo thought NSMBWiiU would move consoles. They were sadly mistaken. Same would go for the GC...
NSMB WiiU failed because there was nothing special about it
Mario games do best when they give the player an exciting new world to explore.
NSMB WiiU was anything but, it was pretty much a rehash of NSMB Wii (which itself was nothing special but benefitted from being the first 2D console Mario in over a decade, the market was ravenous for a new 2D Mario at the time, once that hunger was somewhat satiated, Nintendo needed to wow with its next iteration, it failed to do so)
NSMB on GCN would have done amazing, because of the hunger for a 2D Mario, but it probably would have sucked a lot of steam out of the Wii release (unless Nintendo had decided to put actual effort into the Wii version and not just rehash)
As a result GCN would have sold much better, but Wii sales would probably be about 10-15 million less