I totally understand the procedure you're describing and have attempted the steps per the writeup and your summary of the writeup. There is no gupdate.gcd file to be found on the Garmin, in the /Garmin directory or anywhere else. I've done a search, hidden files are shown, I've even double-checked myself using a command prompt -- no dice.
When I drag the downloaded GCD to the WebUpdater, the process begins and later fails. It is unable to write anything to the Garmin.
Similarly, I cannot get ANY changes I make to the file system to stay after reboot. When I delete the gpx file, it reappears after reboot. When I delete one or more img files, they too return after reboot.
Any changes to the file system cause the system to bring up the screen calibration page upon reboot. I have to hold the power button to force a hard restart. Eventually, that will allow me to boot into the GPS but none of my intended changes have stuck.