Prevent Multiple Form Entries

We are using the ‘Gravity Forms’ plugin but writing to a custom table in our database. Our database is titled ‘wp_dgptp’ and our table is titled ‘dg_application’.

We would like to prevent the user from submitting an entry twice. I.e. if the status is ‘Completed’ in the table then they should get a message telling them they can’t submit again. This is the code:

<?php
    if (get_current_user_id()) {
        //lookup current application:
        $query = "SELECT * FROM dg_application where user_id = " . get_current_user_id();
        $dg_application = $wpdb->get_row( $query, ARRAY_A );
        if ($dg_application["user_id"] && $dg_application["status"] == "Completed") {
            // custom field
            echo get_post_meta($post->ID, "form_complete_message", true);

        } else {
            //let Gravity Forms/WP do thier jobs
            while ( have_posts() ) : the_post();

                // Include the page content template.
                get_template_part( 'template-parts/content', 'page' );

            // End of the loop.
            endwhile;
        }

    } else {
        echo "Sorry. You must be logged in to view this form.";
    }

    ?>

</main><!-- .site-main -->

<?php get_sidebar( 'content-bottom' ); ?>

But unfortunately it’s not displaying the message and users are able to create another entry. Any ideas how to get it working?