Source

File: wp-includes/media.php

function get_post_gallery( $post = 0, $html = true ) {
	$galleries = get_post_galleries( $post, $html );
	$gallery   = reset( $galleries );

	/**
	 * Filters the first-found post gallery.
	 *
	 * @since 3.6.0
	 *
	 * @param array       $gallery   The first-found post gallery.
	 * @param int|WP_Post $post      Post ID or object.
	 * @param array       $galleries Associative array of all found post galleries.
	 */
	return apply_filters( 'get_post_gallery', $gallery, $post, $galleries );
}