WordPress - function to detect if a page/post has a gallery

has-gallery.php

<?php
/**
 * Has Gallery
 *
 * Detect whether the page/post has a gallery.
 *
 * @return boolean True if page/post content has a gallery, else false.
 */
function has_gallery() {
	global $post;

	if ( ! empty( $post ) && is_singular() ) {
		$content = $post->post_content;

		if ( has_shortcode( $content, 'gallery' ) ) {
			return true;
		}
	}

	return false;
}