Untitled

<?php

// update the '2' to the ID of your form
add_filter('gform_pre_render_2', 'total_this');
function total_this($form){

    $form_id = $form['id'];

    foreach($form['fields'] as &$field){

        if($field['inputName'] == 'total-this')
            $total_field_id = $field['id'];

    }

    ?>

    <script type="text/javascript">
    jQuery(document).ready(function($){

        var total = 0;
        var totaled = false;

        $('#gform_<?php echo $form_id; ?>').submit(function(){

            $('.total-this input:checked').each(function(){
                total += parseInt($(this).val());
            });

            jQuery('#input_<?php echo $form_id; ?>_<?php echo $total_field_id; ?>').val(total);

        });

    });
    </script>

    <?php
    return $form;
}