<?php

class Gallery extends Page {

   public static $db = array(
      'SummaryText'=>'Text',
      'GalleryText'=>'Text'
   );
   
   static $has_many = array(
      'Photos' => 'GalleryPhoto'
   );
   
   function getCMSFields() {
      $fields = parent::getCMSFields();
      
      
      $manager = new ImageDataObjectManager(
         $this, // Controller
         'Photos', // Source name
         'GalleryPhoto', // Source class
         'Image' // File name on DataObject
      );
      $manager->uploadFolder = $this->URLSegment;
      
      
      $fields->addFieldToTab('Root.Content.Main', new TextField('SummaryText', 'Summary Text (Appears in the section preview)'), 'Content');
      $fields->addFieldToTab('Root.Content.Main', new TextField('GalleryText', 'Gallery Text (entering anything in here will overwrite any image Titles and Descriptions)'), 'Content');
      $fields->addFieldsToTab("Root.Content.Gallery", array($manager));            
      $fields->removeFieldFromTab('Root.Content', 'StyledText');
      $fields->removeFieldFromTab('Root.Content', 'Column2');
      $fields->removeFieldFromTab('Root.Content', 'Content');
      
      
      return $fields;
      
   }
   
   
}

<?php

class GalleryPhoto extends Photo {

   public static $db = array(
      'HTMLDescription'=>'HTMLText'
   );
   
   static $has_one = array(
      'Gallery' => 'Gallery'
   );
   
   public function getCMSFields(){
      $fields = parent::getCMSFields();
      $fields->removebyname('Description');
      $fields->removebyname('Title');
      $fields->replaceField('HTMLDescription', new SimpleTinyMCEField('HTMLDescription'));
      return $fields;
   }
}