In the GarminCure3 tutorial it's written that CURE firmwares are not bootable (they won't boot and won't flash GUPDATE.GCD, the can only enter the mass storage mode), so you need to flash the ORIGINAL firmware. To do it, select the "ORIGINAL" option, press "Make firmware", press "updater.exe", then put your device into pre-boot mode and flash the firmware with updater.exe.
To put your device into pre-boot mode, do the following:
1) Disconnect your device from the PC.
2) If the device is not turned off, then turn it off forcibly. You'd have to press the power button until the screen becomes completely black.
3) Press and hold the very upper left corner of the screen.
4) Press the power button and immediately connect the device to the PC and wait until updater.exe detects it.