Gutenberg running code only available in front end within shortcode

Use it like this, this defined(‘REST_REQUEST’) will help you to disable running on the backend gutenberg block editor

function generate_new_member_form() {
if(defined('REST_REQUEST')) return;
    ob_start(); ?>
   // a form
   // to generate this form there is some code that uses objects or functions 
   // only available in the front end
  <?php return ob_get_clean();
} 

add_shortcode('new-member-form', 'generate_new_member_form');