<?php
    $gtin = get_post_meta(post_ID,'gtin',true);
?>
<script type="application/ld+json">{
    "gtin13:"<?php echo $gtin;?>"</script>}

"gtin13:"1112223334"

<?php
         function check_gtin(){
         $gtin=get_post_meta(post_ID,'gtin',true);
         if ($gtin!=''){
         echo '$gtin13:$gtin';
         }else{
         echo 'identifier_exits:false';
         }
         }
     ?>
    <script type="application/json+ld">{
        "<?php check_gtin();?>"</script>}

"gtin13:$gtin"

"gtin13:1112223334"