If you want to display the *attached* image gallery, from another post, you can use:

    [gallery id="123"]

where the custom `id` attribute is the post ID.

(no need for custom stuff ;-)

Previous answer:
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If you want to display the *attached* image gallery, from another post, you can use:

    [gallery pid="123"]

where the custom `pid` attribute is the post ID.

This attribute is supported by the following plugin:

    <?php
    /**
     * Plugin Name: Gallery Shortcode with a post ID.
     * Description: Support the post ID attribute (pid) in the gallery shortcode
     * Plugin URI:  http://wordpress.stackexchange.com/a/176937/26350
     */

    add_filter( 'post_gallery', function ( $output, $attr, $instance )
    {
        if(	isset( $attr['pid'] ) )
        {
            global $post;
            $tmp    = $post;
           	$post   = get_post( $attr['pid'] );
            unset( $attr['pid'] );
            $output = gallery_shortcode( $attr );
            $post   = $tmp;
        }
        return $output;
    }, 10, 3 );

Note that you will not be able to order this gallery by hand with drag & drop, since it doesn't contain any explicit image ids. But you can use the `order` and `orderby` attributes instead, for example:

    [gallery pid="123" orderby="date" order="desc"]