You have to create Page template for gallery section.
<?php
/**
* Template Name: Gallery
*
* This is the template that displays all pages by default.
* Please note that this is the WordPress construct of pages and that
* other "pages" on your WordPress site will use a different template.
*
* @package WordPress
* @subpackage Twenty_Sixteen
* @since Twenty Sixteen 1.0
*/
get_header();
?>
// You can write your gallery content here
<section class="section single-page">
<div class="fixed_container">
<?php
// Start the loop.
while ( have_posts() ) : the_post();
// Include the page content template.
get_template_part( 'template-parts/content', 'page' );
// If comments are open or we have at least one comment, load up the comment template.
if ( comments_open() || get_comments_number() ) {
comments_template();
}
// End of the loop.
endwhile;
?>
</div>
</section>
<?php get_footer(); ?>
After that edit you gallery page from admin and assign this template to your gallery page.