Yes Gamecube is only possible By:
* Original Gamecube games
* Burned backups with Cmios ( wiigator with Neogamma highly recommended for the very best compatibilty)
If you have a new Wii you probably cannot play backups from discs.
* WODE-this is a hardware modification. Costs money. But allows Backups from USB.
In order to save gamecube games YOU NEED a GC memory card. They are DIRT cheap!
When playing backups, better get an original GC controller. Their have been many compatibilty problems using 3rd party controllers.
As a side note: When you play GC games on your Wii, your Wii BECOMES a gamecube.
Hardware/software wise. So remotes wont work in GC mode. Only reset, eject and power button on the console itself. Just like a Gamecube.
To be almost wireless, get some wavebirds off e-bay!
It is however, possible to transfer save games from GC memory card to SD card and vice versa in Wii mode with homebrew apps.
The GC memory cards are manageble from the Wii's data management.