@Andy Waltson
Garmin seems to have posted those instructions to help users with V16.10 overcome the possible errors as listed in the Change History from 16.10 to 16.30. The instructions certainly are not intended to help a user 'unbrick' a device because clearly such a device doesn't have MSM access to drop a GCD file into the internal memory. In your case with V16.60 onboard it's not possible to unbrick it with conventional methods because as confirmed by Cnfhbr the firmware is encrypted and therefore defies any attempt to manipulate it in the usual fashions. I would think his (tentative/hesitant) advice to "contact Garmin's technical support, I guess" is the best we can do here unfortunately and he only gave those links as an example of Garmin's attempts to help users revert from a flaky, albeit working, fw rather than as serious help for bricked devices.

@hiban
RGN_Tool reports that Forerunner920XT_990.rgn is "an RGN within an RGN" and upon following this message's direction, i.e.:
Spoiler: small image
[Only registered and activated users can see links. ]


we find the HWID is 1765 after opening that saved fw_all.rgn in RGN_Tool:
Spoiler: image
[Only registered and activated users can see links. ]


This isn't a "usual" Garmin firmware in that there is no boot.bin and the entire fw consists of solely one section, the main (system) software, fw_all.bin, which is flashed to region 14 (hex 0E). The expected boot.bin which is normally contained in that first section, and is actually the ramloader, is missing so the fw flash must be initiated by some other means. I'm so sorry but i personally cannot think of anything further to try and i hope someone else has the knowledge to help you.