GalleryImage.php

<?php

class GalleryImage extends DataObject {
	private static $db = array(
		'Title'             => 'VarChar',
		'ObjectDescription' => 'Text',
		'SortOrder'         => 'Int'
	);

	public static $default_sort = 'SortOrder';

	private static $has_one = array(
		'Image' => 'Image',
		'Page'  => 'Page'
	);

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

		$fields->fieldByName('Root.Main.Image')
		       ->getValidator()->setAllowedExtensions(array('jpg', 'png'));

		$fields->removeFieldFromTab('Root.Main', 'SortOrder');

		return $fields;
	}
}

Page.php

<?php

  /**
   * Base page type
   *
   * @method HasManyList HeaderImages()
   * @method HasManyList GalleryImages()
   */
  class Page extends SiteTree {

    #region Relationships

    /**
     * One to one relations
     * @var array
     */
    private static $has_one = array(
    );

    /**
     * One to many relations
     * @var array
     */
    private static $has_many = array(
        'HeaderImages' => 'GalleryImage',
        'GalleryImages' => 'GalleryImage'
    );

    #endregion Relationships

    #region Public Methods

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

      foreach(array('HeaderImages', 'GalleryImages') as $field) {
        $config = new GridFieldConfig_RelationEditor();
        $config->addComponent(new GridFieldSortableRows('SortOrder'));
        $config->addComponent($uploader = new GridFieldBulkUpload());
        $uploader->setUfSetup('setFolderName', $field);
        $config->addComponent(new GridFieldBulkManager());
        $config->addComponent(new GridFieldGalleryTheme('Image'));
        $fields->addFieldToTab('Root.'.$field, new GridField($field, null, $this->{$field}(), $config));
      }

      return $fields;
    }

    #endregion Public Methods

  }