<?php
/**
 * Returns an array of image IDs stored in gllery meta field
 *
 * @param  string $field_key Gallery field meta key
 * @param  int    $post_id   Post ID to get field from. If not set - will try to use current post.
 * @return array
 */
function my_get_jet_engine_gallery( $field_key = '', $post_id = null ) {

	$gallery = array();

	if ( ! $post_id ) {
		$post_id = get_the_ID();
	}

	if ( ! $post_id || ! $field_key ) {
		return $gallery;
	}

	$meta = get_post_meta( $post_id, $field_key, true );

	if ( ! $meta ) {
		return $gallery;
	}

	$gallery = explode( ',', $meta );

	return $gallery;

}

/**
 * Usage example for field with name '_gallery' and some specific post
 */
$gallery = my_get_jet_engine_gallery( '_gallery', 156 );

foreach ( $gallery as $img_id ) {
	echo wp_get_attachment_image( $img_id );
}