ctrl:ralt_rctrl option is missing in Additional Layout Options
If you say setxkbmap -option ctrl:ralt_rctrl then Right Alt start acting like Right Control, but there is no checkbox in GUI for that option. I guess it should be there.
PS
Quick fix for those how want it: just add ctrl:ralt_rctrl to /org/gnome/desktop/input-sources/xkb-options