Not sure I completely understand what you've tried, but change it in both GCM AND on your device. I've had issues with the backlight on my vaHR that went away after manually changing the setting on both.
ILMETALLO you need to go to https://connect.garmin.com/modern/settings and change Time Format on your profile.
After that everything should be fine. Then after sync your phone via Bluetooth works like charm.
I've had the same issue.;)
