Hi. I had the same problem as you. I could not use AltGr + 2 (@) and similar in Windows.
It seems like the Asus GPU Tweak service "asgt(.exe)" is causing the problem, or at least the programs "keyboard hotkeys" related to it..
Here is a few things you can try:
#1: Open
Asus GPU Tweak and try turning off the "keyboard shortcuts".
You may also have to remove the shortcuts (leave them empty), not only switch the function to "off".
My AltGr would still not work if I just switched the keyboard shortcut function "off"
#2: For me, the GPU Tweak software seems quite bugged in Windows 10 Pro (Norwegian), making me have to go to "regedit" when I want to change something (clock values etc) in the program.
start -> run -> regedit -> go to "HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\ASUS\GPUTweakII\Setting" -> delete all the hotkeys you find there (location may be different on your computer).
This worked the best for me.
#3: You can turn off the asgt service located in Windows services (start -> run -> type "msconfig" -> services -> then disable "asgt")
I am not sure how this effects Asus GPU Tweak in use.
Hope this helps.
I am using:
Asus GPU Tweak2 v1.1.1.1
Windows 10 Pro (Norwegian)
AMD FX6300
Asus Crosshair V Formula-Z
Asus R9 290X DCU2 OC
etc.