importfestivales

Hello
I’m rather new with WordPress but I’m improving. So if you can help me I apreciate it so much.

I have installed the plugin and everything seems to work fine but I don’t know how to show the gallery in the front end. My single.php file look like this

<?php get_header(); ?>
<section id="content" class="portada">
	<ul class="sidebar-categorias">
    	<?php if ( !function_exists('dynamic_sidebar') || !dynamic_sidebar('Inicio Sidebar') ) : ?>
        <?php endif; ?>
    </ul>
	<div id="cuerpo">
		<?php if (have_posts()) : ?>
		<div id="archive">
			<?php while (have_posts()) : the_post(); ?>
			<div class="single-item">
				<div class="thumbnail">
                	<a class="titulo_archivo" href="<?php the_permalink(); ?>" style="text-decoration: none !important;">
<?php the_post_thumbnail('medium'); ?>
                    </a>
                    <div class="clearBoth"></div>
                </div>
                <div class="text">
                	<h2> <?php the_title(); ?> </h2>
                    <p> <strong> Descripci&oacute;n:</strong></p>
					<?php the_content(); ?>
                    <p> <strong>Precio:</strong> <?php echo $variable = get_post_meta($post->ID, 'precio', true); ?></p>
                </div>
                <div class="clearBoth separator"></div>
			</div><!-- end item -->
			<?php endwhile; ?>
		<?php else : ?>
		<div id="archive">
			<div class="item">
				<p>Lo siento, no hay artículos en esta categoría.</p>
			</div>
		<?php endif; ?>
        	<div class="clearBoth"></div>
		</div><!-- end archive -->

		<ul id="sidebar">
			<?php if ( !function_exists('dynamic_sidebar') || !dynamic_sidebar('Blog Sidebar') ) : ?>
            <?php endif; ?>
		</ul><!-- end sidebar -->
	</div>
	<div style="clear:both"></div>
</section>
<?php get_footer(); ?>

I know that the part that shows the featured image is <?php the_post_thumbnail('medium'); ?> but I don’t know what function I have to put there.

thank you so much for your help