I used to have this workaround in my system configuration `services.dbus.packages = [ pkgs.gnome3.dconf ];`
Copied from issue https://github.com/rycee/home-manager/pull/436#issuecomment-449755377
Week ago after update I've received the same GDBus error.
Enabling dconf fixed it `programs.dconf.enable = true;`
Looks like dconf knows how to add itself to dbus and systemd https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/programs/dconf.nix#L55-L57