The Alt-GR key is not the same as the Alt key, when you press the Alt-GR key, that indicates to Windows you have pressed Alt + Ctrl keys, it is used for some specific functions in Windows like special characters on your keyboard.
Windows does not have any method or setting to re-map keys, if you want to remap the Alt-GR key to the Alt key you will need to use an app for that, the Microsoft PowerToys app available on the Microsoft Store can remap keys in the Keyboard Manager section.