I've already checked the option section in
/usr/share/X11/xkb/rules/base.lst but I didn't find an entry for AltGr...
Surely, you can achieve this by editing the xkb rules set but it might be more convenient to use
You can use
xev to lookup the keycode of your AltGr key. On my system it also has the name ISO_Level3_Shift which is already used for the modifier mod5 (use
xmodmap to see the modifier maps). Removing it from mod5 and adding it to the control modifier makes the AltGr key work like the Ctrl:
xmodmap -e "remove mod5 = ISO_Level3_Shift"; xmodmap -e "c
add control = ISO_Level3_Shift".