You need a gamecube homebrew compatible cMIOS. The recommend cMIOS for NeoGamma should do it fine.
Then you need a way to boot GC .dols with that cMIOS. NeoGamma in GC mode external, the SD-Boot .dol as sd/NeoGamma/GC.dol on a sd card in the wii's sd card slot, and a retail GC disc in the drive should do it. Or just use some GC homebrew launcher, which still requires the cMIOS, but might be easier to use. Also with a GC homebrew launcher it should work with a retail wii disc in the drive too.
But don't ask me about the .iso format you need, or if the .isos need to be defragmented.