As mentioned in the other thread, GNOME manages the keyboard layout and relevant settings itself and the services.xserver.{layout,xkb*} NixOS options are only used for initial set up.
Your issue sounds like you have another layout added in GNOME – try clicking the language panel in the top bar to see them.
Currently, the least foot-gunny option for setting the GNOME keyboard layout declaratively is using home-manager as mentioned in Problem with `xkbOptions` - it doesn't seem to take effect - #12 by jtojnar.