$error_message isn't showing when checkbox is empty

<form name="contactform" method="post" action="send_form_email.php">

<!--Name input-->
<input placeholder="Full Name *" type="text" name="name" maxlength="50">
<br />      

<!--Email input-->
<input placeholder="Email Address *" type="email" name="email" maxlength="80">
<br />

<!--Phone number input (not required)-->
<input placeholder="Telephone Number" type="tel" name="telephone" maxlength="15">
<br />

<!--Company name input (not required)-->
<input placeholder="Company" type="text" name="company" maxlength="50">
<br />

<!--Comments & messege input-->
<textarea  placeholder="Please include as much information as possible *" name="comments" maxlength="500" cols="25" rows="5"></textarea>
<br />

<!--Check for privacy policy-->
<label class="GDRP">
I consent to having this website store my submitted information so they can respond to my inquiry.
  <input type="checkbox" name="GDRP">
</label>

<!--Submit button-->
<input type="submit" value="SUBMIT">

</form>

This is my HTML - I don't know how important it is to helping me find a solution but I figured, why not. I tried cleaning it up so you can get through with helping me as quick as possible. The only important fields are the 'name', 'email', 'comments' and the question at matter 'GDRP'.

<?php if(isset($_POST['email'])) {

                // EDIT THE 2 LINES BELOW AS REQUIRED
                $email_to = "email@adress.com";
                $email_subject = "www.Adress.com - CONTACT FORM";


                function died($error) {
                    // error code can go here
                    echo "We are very sorry, but there were error(s) found with the form you submitted: ";
                    echo $error."<br />";
                    echo "Please fix these errors.";
                    die();
                }

                // validation expected data exists
                if(
                    !isset($_POST['name']) ||
                    !isset($_POST['email']) ||
                    !isset($_POST['company']) ||
                    !isset($_POST['telephone']) ||
                    !isset($_POST['comments']) ||
                    !isset($_POST['GDRP'])
                ) { 
                died('We are sorry, but there appears to be a problem with the form you submitted.'); 
                }

                $name = $_POST['name']; // required
                $email_from = $_POST['email']; // required
                $telephone = $_POST['telephone']; // not required
                $company = $_POST['company']; // not required
                $comments = $_POST['comments']; // required
                $GDRP = $_POST['GDRP']; // required

                $error_message = "";
                $email_exp = '/^[A-Za-z0-9._%-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,4}$/';
                $string_exp = "/^[A-Za-z .'-]+$/";
              if(!preg_match($string_exp,$name)) {
                $error_message .= '<br /><br /><font color="red">Full Name</font><br /><br />';
              }
              if(!preg_match($email_exp,$email_from)) {
                $error_message .= '<font color="red">Email Address</font><br /><br />';
              }
              if(strlen($comments) < 2) {
                $error_message .= '<font color="red">Comments</font><br /><br />';
              }
              if(!empty($GDRP)) {
                $error_message .= '<font color="red">GDRP Agreement</font><br /><br />';
              }

              if(strlen($error_message) > 0) {
                died($error_message);
              }
                $email_message = "Form details below.\n\n";

                function clean_string($string) {
                  $bad = array("content-type","bcc:","to:","cc:","href");
                  return str_replace($bad,"",$string);
                }

                $email_message .= "First Name: ".clean_string($name)."\n";
                $email_message .= "Email: ".clean_string($email_from)."\n";
                $email_message .= "Company: ".clean_string($company)."\n";
                $email_message .= "Telephone: ".clean_string($telephone)."\n";
                $email_message .= "Comments: ".clean_string($comments)."\n";


            // create email headers
            $headers = 'From: '.$email_from."\r\n".
            'Reply-To: '.$email_from."\r\n" .
            'X-Mailer: PHP/' . phpversion();
            @mail($email_to, $email_subject, $email_message, $headers);  
        ?>

        <body>
        Thank you for contacting us. We will be in touch with you as soon as possible.
        </body>

        <?php
        }
        ?>

My problem is that the checkbox ('GDRP') isn't showing $error_message when it's not filled in. In fact, none show when 'GDRP' is empty. If you fill 'GDRP' but leave the others [required fields] empty all of their $error_message except for 'GDRP' will show.