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 know).
    Some homebrew games can read and write on SD cards because they are using their own programming calls to access the memory cards.