// HomePage
class HomePage extends Page {
   static function isLocalhost() {
      return Utils::isLocalHost();
   }

   public static $has_one = array(

   );
   
public static $has_many = array(
"Thumbnails" => "Thumbnail",
);

   function getCMSFields() {
      $fields = parent::getCMSFields();

      $gridFieldConfig = GridFieldConfig_RecordEditor::create()->addComponents(new GridFieldDeleteAction('unlinkrelation'));
      $gridFieldConfig->addComponent(new GridFieldSortableRows('SortOrder'));
      $gridField = new GridField("Thumbnails", "Thumbnails", $this->Thumbnails(), $gridFieldConfig);
      $fields->addFieldToTab("Root.Thumbnails", $gridField);      
      return $fields;
   }
}

// Thumbnails
<?php

class Thumbnail extends DataObject {
   public static $db = array(
      'Name' => 'Text',
      'Caption' => 'Text',
      'SortOrder'=>'Int'
   );

public static $has_one = array(
   "Image" => "Image",
"HomePage" => "HomePage",
"LinkedPage" => "Page"
);

// Summary fields
public static $summary_fields = array(
'Thumbnail' => 'Thumbnail',
'Name' => 'Name',
'Caption' => 'Caption'
);

public function getThumbnail() {
return $this->Image()->CMSThumbnail();
}
}