Control script generated buttons with a Gamepad

How can I control script generated buttons with a gamepad? until today I have been using GUI.FocusControl(MenuString); in combination of an int switch to move through the different menu buttons the problem is that method requires to know before hand how many buttons are there so it doesn't work with buttons generated by a script for example this code I have to display the items in inventory:

 if (MenuString == "Items"){
         for (int y = 0; y < _rows; y++ ){
             for (int x = 0; x < _cols; x++ ){
                 GUI.Box (new Rect(x*(Screen.width/15),y * (Screen.height / 8.5f), Screen.width/15 ,Screen.height / 8.5f),new GUIContent (InventoryScript.itemsInInventory[x + y * _cols].Icon));
                 GUI.Label (new Rect(x*(Screen.width / 2f),y*(Screen.height/ 4)*0.25f, Screen.width/15 ,Screen.height / 8.5f)," " + InventoryScript.itemsInInventory[x + y * _cols].Cantidad,InventoryStile);
                 }
             }
         }

How can I do this?

Thanks!