I'm a bit hazy on exact reasoning for the different terminology, other than maybe for easy id of files, but basically there's no difference between the same gcd files. A file named, for example, 'nuvi16xxEU_regionfileonly__390.gcd' is meant to be used with WebUpdater or GarminExpress to be uploaded to the unit via that software and it will be placed on the device as GUPDATE.GCD. You can do the same yourself by renaming 'nuvi16xxEU_regionfileonly__390.gcd' to 'gupdate.gcd' and plonking it in the 1690's .System folder. On the next boot cycle the firmware will update regardless of how gupdate.gcd was put there.

I personally prefer to convert the gcd file to an rgn file using RGN_Tool and upload it in preboot using updater.exe.

The 'boot.bin' (self-explanatory name) is but one major component of the firmware, the other 2 in the 1690 fw are 'fw_all.bin' and 'fw1_resources.bin' and all together make up the *.gcd and *rgn files. You can look closer at fw using a hex editor such as XVI32:
Code:
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If you want to get into fw in-depth, best to google for some tech pages.

Other 1xxx devices will boot with the internal memory completely empty, but the 1690/1695 models are quite different to the others. I'd reformat again with RMPrepUSB, reload the files from your backup and reboot. My AU 1690 sticks on logo on about every third or fourth boot, but always boots up when forced off and restarted. They really are flaky units as i said and i consider them to be problematic.