single.php

<?php

/**
 * Related Posts by category
 *
 * @author Bill Erickson
 * @see https://www.billerickson.net/custom-wordpress-queries/
 */
function be_related_posts_by_category() {
  
  $categories = get_the_terms( get_the_ID(), 'category' );
  $category = array_shift( $categories );
  
  $loop = new WP_Query( array(
    'posts_per_page'  => 5,
    'category_name'   => $category->slug,
    'post__not_in'    => array( get_the_ID() )
  ) );
  
  if( $loop->have_posts() ):
    echo '<div class="related-posts">';
    echo '<h3>Related Posts</h3>';
    while( $loop->have_posts() ): $loop->the_post();

      echo '<div class="related-post">';
      if( has_post_thumbnail() )
        echo '<a class="entry-image-link" href="' . get_permalink() . '">' . get_the_post_thumbnail( get_the_ID(), 'medium' ) . '</a>';
      echo '<h5 class="entry-title"><a href="' . get_permalink() . '">' . get_the_title() . '</a></h5>';
      echo '</div>';
  
    endwhile;
    echo '</div>';
  endif;
  wp_reset_postdata();
}
add_action( 'genesis_after_entry', 'be_related_posts_by_category' );