Have you tried formating the card in the camera?
This will most likely format the card to fat16 which isn't really what the gp2x wants, but it will atleast let you test that the card does work in the camera. If the camera is unable to format the card, then you card is most likely bad. IF you can format the card in the camera, and use it, then you should get a USB card reader and use it + windows xp to format the card to Fat32.