You are not supposed to edit files from packages on classic Linux distros, that will just lead to a mess. The immutable nature of Nix store prevents you from doing that, which makes tinkering a bit harder but at least it removes the temptation.
You could still override the gsettings-desktop-schemas package when building your system but that would end up rebuilding every package that depends on it, which is everything depending on GTK. Or you can replace transplant your overridden package with system.replaceRuntimeDependencies NixOS option to avoid the rebuilds but that is ugly hack.
But since the GDM display manager responsible for the login screen runs as a separate gdm user, configuring its settings is a bit complicated.
There are actually two proper options for changing the settings as a user:
Updating dconf database.
This is what ends up happening when you change the settings in GNOME Settings.
Typically, each user has their own dconf database inside their home directory (~/.config/dconf/user).
But gdm’s home directory is /run/gdm which will not persist when system is shut down so you cannot just copy the database there.
You could create a systemd service that would populate the dconf database each time gdm user logs in. This is actually what home-manager does so you might want to use it.
NixOS is not magic, it just automates what on different distros is often imperatively managed. So just like there, if you change a dconf database, it will not have an effect on completely unrelated file.
Also nixos-rebuild does not manage your home directory. You can use home-manager as a NixOS module and then it can modify the content of your home directory as a part of an activation script but that is opt-in on per-feature basis – you probably would not like it if it suddenly started overwriting random files in your home directory.