I have the Gecko and it can be used for just a
couple of things.
Homebrew Games for Gamecube can
be run from it using SDLoad in combination with an Action Replay and a crazy
long code. I've done this.
Animal Crossing 2? for the
Japanese market offered the ability to take screen shots and save them directly
to the SD card so they could be viewed and shared through your computer. I have
not done this.
My understanding of why standard
game saves don't work is SD cards and the gamecube memory card while
electrically are very similar they use a different "Command Set" for reading and
writing to their memory. At a really basic level (abstraction leaks) it's like
IDE and SCSI both are harddrives that read and write data but they are not
interchangable because their command set is different (and their wiring, I
Some homebrew games can read and
write on SD cards because they are using their own programming calls to access
the memory cards.