You're supposed to go missed if you have a RAIM or GPS failure message.
But all the GPS IAP's I've seen have missed approaches that require
use of the GPS to fly correctly (fly to this waypoint, then go to this
other waypoint and hold). So what do you do? Use the GPS to fly the
missed despite the RAIM failure (assuming the whole GPS hasn't gone
belly up)? Call ATC for help?