add gc

nixos/configuration.nix

@@ -7,17 +7,20 @@
 {
   imports = [ ];
 
-  nix.settings.experimental-features = [
+  nix = {
+    settings.experimental-features = [
       "nix-command"
       "flakes"
-
     ];
-  nix.gc = {
+
+    gc = {
       automatic = true;
       persistent = true;
       dates = [ "daily" ];
       options = "--delete-older-than 7d";
     };
+  };
+
   nixpkgs.config.allowUnfree = true;
 
   # Secret Management