Thanks for suggesting getallheaders(). With your help I found two ways, with and without Javascript.

/* Add GPC Banner to WP Header */
function display_gpc_signal_banner() {
echo '<section id="headerBannerGpc" class="bg-color"><div 
class="container detect-gpc-signal py-1 small"><span class="alert- 
circle"></span> Global Privacy Signal Detected</div></section>';
}
add_action('wp_head', 'display_gpc_signal_banner');

// Detect GPC Signal with Javascript
function detect_gpc_signal_js() {
?>
  <script>
  if(navigator.globalPrivacyControl == true ) {
    // Add a class to the body tag -- This makes the GPC banner visible
    document.body.classList.add('gpc-signal-detected');
    document.getElementById('headerBannerGpc').style.display = "block";
  } else {
    // Add a class to body tag declaring the GPC signal was not detected
    document.body.classList.add('no-gpc-signal-detected');
    document.getElementById('headerBannerGpc').style.display = "none";
  }
  </script>
  <?
}
add_action('wp_footer', 'detect_gpc_signal_js');

// Add a unique class to the body tag when GPC signal is detected
function detect_gpc_signal_body_class( $classes ) {
$headers = getallheaders();
if($headers['sec-gpc'] == 1) {
  $classes[] = 'gpc-signal-detected-no-js';
} else {
  $classes[] = 'no-gpc-signal-detected-no-js';
}
return $classes;
}
add_filter( 'body_class', 'detect_gpc_signal_body_class' );

renew

The original function will work when HTTP_ is added to the value.

if (isset($_GET['HTTP_SEC_GPC'])) {
  echo 'GPC signal detected in GET data';
  } elseif (isset($_POST['HTTP_SEC_GPC'])) {
  echo 'GPC signal detected in POST data';
  } elseif (isset($_COOKIE['HTTP_SEC_GPC'])) {
  echo 'GPC signal detected in COOKIE data';
  } elseif (isset($_SERVER['HTTP_SEC_GPC'])) {
  echo 'GPC signal detected in SERVER data';
  } else {
   echo 'No GPC signal detected';
  }