// Snippet to change GravityForms 'Submit' button to 'Processing...' and disable it
add_filter('gform_pre_render_1', 'disable_submit');
function disable_submit($form) {
    ?>

    <script type="text/javascript">
    jQuery(document).ready(function($){
        $('#gform_submit_button_<?php echo $form['id']; ?>').on('click', function(event){

            var submitCopy = $(this).clone();
            submitCopy.prop('id', '').prop('disabled', true).prop('value', 'Processing, please wait...').insertAfter($(this));

            $(this).hide();

        });
    });
    </script>

    <?php
    return $form;
}