I think maybe your problem could be solved by now, but I would like to post my solution if you, or other Gigabyte users need it.
Your method is correct, so it might just be registry settings problem. Here is the way I set mine up:
Log on as Administrator
Display the GUI and make your configuration, then select "save" by clicking the buttons at the bottom of the dialog box.
In the tray area, right-click on the ET6 icon, and TICK: "Always Run on Next Reboot".
Open Regedit and inspect key "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\RunOnce". Remember the command line for the program ETCall in value "EasyTune VI".
In the tray area, right-click on the ET6 icon and:
UNTICK "Always Run on Next Reboot"
TICK: "Auto load last save on next reboot" (The saved settings are stored in registry key "HKEY_CURRENT_USER\Software\Gigabyte\EasyTune6", and will be loaded when ETCall starts up).
Add a setting to start the ETCall command line (from point 5 above) when the computer starts. On my computer I use a special service application to start the ETCall command line, but it should work equally well using Scheduled Tasks as you did, and selecting to start the task on boot.
Make sure the scheduled task starts with the admin account credentials you made the configuration with.
Reboot the computer and log in as a normal user. Wait for all startup programs to load, it takes a while to kick in on my PC, but not more than a few minutes.
Normal users cannot load the Easy Tune 6 GUI, but you can log in as Administrator to update the configuration at any time. The important thing for me was to get the fans under control when a normal user logged in.