In GNOME, you should use xkb-options setting from the org.gnome.desktop.input-sources GSettings schema.
See Strange xkbOptions behavior (GNOME) - #5 by jtojnar for an example how to set GSettings keys directly in a dconf database using NixOS option.
You can also set the value in the xserver config, since that is needed if you want to apply it to TTY with console.useXkbConfig, and then refer to it within the dconf definitions similarly to how it is done here Setting caps lock as ctrl not working - #5 by jtojnar