The GCD contains only a bootloader (section 0505 - region 5) rather than a ramloader aka boot.bin (section 0800; region 12 [hex 0C]). Usually, the ramloader is flashed to region 12 which is a virtual region and then the bootloader component in it is then flashed internally from 12 to region 5 along with the X-loader to region 43. The watches are set up quite differently and i'm unsure of how the firmware is handled by the device because in most Garmin devices region 5 is protected from direct flashing.

You can certainly try converting the fenix's GCD to an RGN using RGN_Tool (or better, with [Only registered and activated users can see links. ]) however either as a GCD or RGN the file cannot be processed by Cure3 because it doesn't contain a boot.bin.

You could try using QuickCure3. It's a modified 'universal' ramloader of sorts. Many devices are compatible with it, but some are not unfortunately. It may work or at worst it might hard-brick the watch. On second thoughts, it probably won't work. Likely because such devices don't have an operative region 12.