Floppy Disks, Zip Disks, VHS, Beta, Cassette, Mini DV (IIRC) and Hard drives. These are the only creatures that fear the magnet.
The fear comes from the fact that their data is actually written on a form of magnetic media. If it's solid state like a flash card (ie SD cards and DS games), there's only magical writing involved, not magnetic spinning.
FYI: if the GBA cart has a battery save, magnets won't hurt those either.