add_action('pmxi_saved_post', 'update_gallery_ids', 10, 3);
function update_gallery_ids($id) {

	$attached_images = get_attached_media( 'image', $id );

	// Prepare images into array
	$arr = array();
	foreach($attached_images as $image)
		array_push($arr, $image->ID);

	// Prepare shortcode
	$string = '<p>[gallery ids="'. implode(", ",$arr) . '"]</p>';

	// Prepare post for updating
	$content_post = get_post($id);
	$content = $content_post->post_content;
	$my_post = array();
	$my_post['ID'] = $id;
	$my_post['post_content'] = $content . $string;

	// Save post
	wp_update_post( $my_post );

}