Show or hide the featured image thumb if post has gallery

single.php

<?php
// if the post does NOT have a [gallery], show the featured image
if ( strpos( $post->post_content,'[gallery' ) === false ) {
  $gallery = 0;
  if ( has_post_thumbnail() && 'gallery' != get_post_format() ) {
    $large_image_url = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'large' );
    if ( ! empty( $large_image_url[0] ) ) {
      echo '<a rel="lightbox" href="' . esc_url( $large_image_url[0] ) . '" title="' . the_title_attribute( array( 'echo' => 0 ) ) . '">';
      echo get_the_post_thumbnail( $post->ID, 'thumbnail', array( 'class' => 'float-right thumbnail' ) );
      echo '</a>';
    }
  }
} else {
  $gallery = 1;
  // Yes, there's a gallery so don't call in the featured image. simples.
}
?>