Custom Post Previous and Next functionality

single-gallery.php

<?php

/*
    Template Name: Portfolio
*/

function curPageURL() {
    $pageURL = 'http';
    
    if ($_SERVER["HTTPS"] == "on") {
        $pageURL .= "s";
    }
    
    $pageURL .= "://";
    
    if ($_SERVER["SERVER_PORT"] != "80") {
        $pageURL .= $_SERVER["SERVER_NAME"].":".$_SERVER["SERVER_PORT"].$_SERVER["REQUEST_URI"];
    } else {
        $pageURL .= $_SERVER["SERVER_NAME"].$_SERVER["REQUEST_URI"];
    }
    return $pageURL;
}

    if ( have_posts() ) {
        while ( have_posts() ) {
            the_post();
            $postID = get_the_ID();
        }
    }
    wp_reset_query();

    $wp_query = new WP_Query( array(
        'post_type' => 'gallery',
        'posts_per_page' => 1000
    ));
        
    if( have_posts() ) {
    
        $i = 0;
        while( $wp_query->have_posts()) {
            $wp_query->the_post();
            $links[$i] = get_permalink();
            $i++;
        }
    }
    wp_reset_query();
    wp_reset_postdata();  
    
    $currentURL = curPageURL();
    
    if(  count($links) > 0 ) {
        $commaSeparated = implode(',', $links) . $currentURL;
        $linksCount = count($links) - 1;
        
        for( $i=0; $i <= $linksCount; $i++ ) {
            if( $links[$i] == $currentURL ) {
                $previousItemNumber = $i - 1;
                $currentItemNumber = $i;
                $nextItemNumber = $i + 1;
                break;
            }
        }
        
        if( strlen( $links[$previousItemNumber] ) > 0 ) {
            $previousLink = '<a id="previousGallery" href="' . $links[$previousItemNumber] . '" target="_self">Previous</a>';
        }
        else {
            $nextLink = '';
        }
        
        if( strlen( $links[$nextItemNumber] ) > 0 ) {
            $nextLink = '<a id="nextGallery" href="' . $links[$nextItemNumber] . '" target="_self">Next</a>';
        }
        else {
            $nextLink = '';
        }
        
        if( strlen( $links[$nextItemNumber] ) > 0 && strlen( $links[$previousItemNumber] ) > 0 ) {
            $centerElement = '<div id="centerElement"></div>';
        }
        else {
            $centerElement = '';
        }
    }
    
?>    

<?php get_header(); ?>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
<script src="<?php bloginfo('template_url'); ?>/js/slides.min.jquery.js"></script>
<script>
    $(function(){
        $("#slides").slides();
    });
</script>
<article id="portfolioPage" class="page">
    <h3 class="pagetitle portfolioTitle">
    </h3>
    <div id="galleryNavigation">
        <? echo $previousLink; ?>
        <? echo $centerElement; ?>
        <? echo $nextLink; ?>
    </div>
    <?php
    $wp_query = new WP_Query( array(
        'post_type' => 'page',
        'posts_per_page' => 1,
        'p' => $postID
    ));
    ?>    
    <?php if (have_posts()) : ?>
    <?php while ($wp_query->have_posts()) : $wp_query->the_post(); ?>

    <div id="galleryContainer">
      <?php 
          $gallery_id = get_post_meta($post->ID, 'gallery', true);
          if(empty($gallery_id))
          {
            $gallery = 1;
          }
          elseif(!empty($gallery_id))
          {
            $gallery = $gallery_id;
          }
          echo do_shortcode('[nggallery id=' . $gallery . ']');
      ?>
    </div>
    <div class="content">
        <?php the_content(); ?>
    </div>
    <?php endwhile; endif; ?>
</article>

        
<div id="galleryItems">
    <?php
        $wp_query = new WP_Query( array(
            'post_type' => 'gallery',
            'posts_per_page' => 100
        ));
    ?>

    <?php if (have_posts()) : ?>
    <?php while ($wp_query->have_posts()) : $wp_query->the_post(); ?>

    <div class="galleryItem">
        <a href="<?php echo get_permalink(); ?>" target="_self" title="<?php the_title(); ?>">
            <?php  if((get_post_meta($post->ID, "thumbnail", true))) { ?>
                <img src="<?php echo get_post_meta($post->ID, 'thumbnail', true); ?>" height="100" width="150" />
            <?php } ?>
            <span><?php the_title(); ?></span>
        </a>        
    </div>


    <?php endwhile; ?>
    <?php else : ?>
       
    <?php 
        endif; 
        wp_reset_query();
    ?>
</div>
<?php get_footer(); ?>