get_post_gallery_images() │ WP 3.6.0

$gal = get_post_gallery_images(2179);

/* $gal will contain
Array
(
	[0] => http://example.com/wp-content/uploads/2016/02/image12-80x80.png
	[1] => http://example.com/wp-content/uploads/2016/02/image11-80x80.png
	[2] => http://example.com/wp-content/uploads/2016/02/image10-80x80.png
)
*/

In the block editor (Gutenberg), the gallery has a different appearance-not a shortcode, so the function get_post_gallery_images() will return an empty array.

A temporary solution:

/**
 * Returns all picture URLs of the first gallery from the specified post text.
 * Returns the result if the gallery exists and is inserted as a block (Gutenberg).
 *
 * @param int|WP_Post $post
 *
 * @return array
 */
function get_post_block_gallery_images( $post = 0 ) {
	$post = get_post( $post );

	// Post not found - return empty array
	if ( ! is_a( $post, 'WP_Post' ) ) {
		return [];
	}

	// The "Gallery" block was not found - return an empty array
	if ( ! has_block( 'gallery', $post->post_content ) ) {
		return [];
	}

	// Search for all blocks in the content
	$post_blocks = parse_blocks( $post->post_content );

	// Go through all the blocks looking for the right one
	foreach ( $post_blocks as $block ) {
		// Search for the "Gallery" block with the passed IDs of the images
		if ( $block['blockName'] === 'core/gallery' && ! empty( $block['attrs']['ids'] ) ) {
			return array_map( function ( $image_id ) {
				// Get a link to the original image
				return wp_get_attachment_image_url( $image_id, 'full' );
			}, $block['attrs']['ids'] );
		}
	}

	// If the "Gallery" block is not found - return an empty array
	return [];
}

A simple example of how to append the raw image URLs to the content of any post or page that has at least one gallery.

add_filter( 'the_content', 'wpdocs_show_gallery_image_urls' );

/**
 * Show image URLs below the content
 */
function wpdocs_show_gallery_image_urls( $content ) {

	global $post;

	// Only do this on singular items
	if( ! is_singular() )
		return $content;

	// Make sure the post has a gallery in it
	if( ! has_shortcode( $post->post_content, 'gallery' ) )
		return $content;

	// Retrieve the first gallery in the post
	$gallery = get_post_gallery_images( $post );
	$image_list = '<ul>';

	// Loop through each image in each gallery
	foreach( $gallery as $image_url ) {
		$image_list .= '<li>' . '<img src="' . $image_url . '">' . '</li>';
	}
	$image_list .= '</ul>';

	// Append our image list to the content of our post
	$content .= $image_list;

	return $content;
}