Code snippet for this: https://docs.gravityview.co/article/468-rest-api (Authentication section)

gv-rest-api-nonce.php

<?php 
$entries_url = 'https://try.gravityview.co/demo/wp-json/gravityview/v1/views/148/entries.json';
$nonce = wp_create_nonce( 'wp_rest' );
?>
    <script type="text/javascript">
        jQuery.ajax( {
            url: <?php echo json_encode( $entries_url ); ?>,
            method: 'GET',
            beforeSend: function ( xhr ) {
                xhr.setRequestHeader( 'X-WP-Nonce', <?php echo json_encode( $nonce ); ?> );
            },
        } ).done( function ( response ) {
            console.log( response );
        } );
    </script>
<?php