Thank you all for answers. I was unable to solve this using AutoHotkey — PhilLho’s answer was close, but I really needed exatly the same behaviour as with left Alt key.
However, the registry thing actually worked as I needed.
Save this as AltGR_to_LeftAlt.reg file and run it:
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Keyboard Layout]
"Scancode Map"=hex:00,00,00,00,00,00,00,00,02,00,00,00,38,00,38,e0,00,00,00,00
Or, there is a GUI tool that does this for you — it’s called SharpKeys and works peachy:
Oh, and don’t forget to reboot or log off — it won’t work until then!