Various people in rec.boats.electronics report odd problems, patches applied etc. with Garmin, Magellan units.
It is unlikely that the position reported by GPS receivers will be significantly in error. However, units may lose lock or take more time to acquire satellites, and not give a fix for ten minutes or longer. It is therefore not a good time to connect the autopilot to the GPS and go below to cook dinner (you are, of course, obliged to keep a proper watch in any case..)
See the following sites for a brief technical description:
See also this list of affected systems reported by Marc Brett (Joe.Mehaffey.com)
Many GPS products must have a firmware upgrade to be Y2K/rollover compliant. This usually means obtaining a ROM chip from the manufacturer or dealer, opening up the device and swapping the chip. Some devices may be upgradeable by connecting them to a PC and running a special program, while others may require unsoldering parts and should be left to a specialist in most cases to retain product warranty.
If you have any information on other systems, or corrections to this list, please let me know ()