AFAIK, Vim (or gvim) cannot tell apart the left and right Ctrl keys, nor can it tell apart Ctrl- and Ctrl-Shift- with A-Z (and possibly @ [ \ ] ^ _). Similarly, AltGr and plain Alt are told apart, not by Vim itself but by some interface beween it and the keyboard (possibly including the console emulator in Vim but not in gvim).

For mappings, IMHO the best choices for a {lhs} are the F1 to F12 keys, then Shift-F1 to Shift-F12 if the console doesn't "steal" them (or in gvim); also any characters not put to use by (g)Vim present on your keyboard (for instance ² ³ £ µ § é è ç à ù on my Belgian keyboard). This allows you to program (in your vimrc) a shortcut for any keycode or key combination which you find difficult to produce directly.

For instance, the [ on my AZERTY keyboard is the key to the right of the P, when used with AltGr. For a long time I didn't know how to produce a keycode which gvim would recognize as Ctrl-[ (to go to a tag in its help) so I added

      map <F9> <C-[>

in my vimrc. Now if I want to follow a hotlink to a help tag in gvim, presto! place the cursor on it, then hit F9. (Double-click is another possibility but the above mapping uses the keyboard.)

Best regards,
Tony.

It might be possible to detect this on xterm using the xterm-modifier-key feature (:h xterm-modifier-keys) or the kitty protocol (:h kitty-keyboard-protocol). Not sure if either of those allow to detect Alt_Gr and they only work with specific terminals.