In your case you seem to be assuming that a corrupt GCD is the problem and often after a firmware update using GarminExpress or WebUpdater, or simply manually placing a later version GCD in the ....
This is why it's safer to update via a gupdate.gcd in a Garmin folder on a media card because if the GCD is faulty it still 'bricks' the device but then removing the card allows the device to boot.
TLDR: You must load cure firmware so you can delete suspect files progressively and as a last resort re-format.