I want to display all related posts for a selected tag in WordPress

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You’re limiting your query via posts_per_page to five results, to get all possible results use -1. Don’t use $wp_query as variable name, as it is associated to a global variable with the same name, which can lead to interferences. But that said, you absolutely don’t need a custom query in your tag.php. You are already querying for a specific tag, with ?tag=abc or /tag/abc in your URL. WordPress catches that and selects the correct template, and limits the query to that tag. There is a hierarchy on how tag template (codex page; read it for more information) files get selected:

  1. tag-slug.php
  2. tag-id.php
  3. tag.php
  4. archive.php
  5. index.php

If available then the order is first to fifth, if not WordPress is looking for the next one. Additionally I would recommend to give the Theme Developer Handbook a good read, especially the section about Template Files. In the end you just need a template file to fit your needs. Below a exemplary template file.

<?php 
/**
 * The template for displaying tag pages.
 *
 * @package WordPress
 * @subpackage Theme
 */ 
?>
<?php get_header(); ?>

<!-- tag | start -->
<div class="row">
  <div class="col-xs-12">
    <?php the_archive_title( '<div class="page-header"><h1 class="page-title">', '</h1></div>' ); ?>
  </div>
</div>
<div class="row">
  <div class="col-xs-12">
    <?php if ( have_posts() ) : ?>
    <?php $description = get_the_archive_description(); ?>
    <?php if ( ! empty( $description ) ) :?>
    <div class="archive-description">
      <?php echo $description; ?>
    </div>
    <?php endif; ?>
    <?php while ( have_posts() ) : the_post(); ?>
    <!-- tag loop | start -->
    <div id="content" <?php post_class(); ?>>
      <div class="row">
        <div class="col-xs-12">
          <?php the_title( '<div class="post-header"><h2 class="post-title">', '</h2></div>' ); ?>
          <?php if ( has_post_thumbnail() ) : ?>
          <div class="entry-thumbnail">
            <?php the_post_thumbnail(); ?>
          </div><!-- .entry-thumbnail -->
          <?php endif; ?>
          <div class="entry-content">
            <?php the_excerpt(); ?>
          </div><!-- .entry-content -->
        </div>
      </div>
    </div><!-- #content -->
    <!-- tag loop | end -->
    <?php endwhile; ?>
    <?php endif; ?>
  </div>
</div>
<!-- tag  | end -->
<?php get_footer(); ?>