edited for no priority and action

// http://www.gravityhelp.com/forums/topic/display-result-variable-in-confirmation-message
// consider running only on a specific form by changing the "7" is your form ID
add_action("gform_pre_submission_7", "send_post");

// there is no entry in the pre_submission hook
function send_post($form)  {

    // dump the $form object to get the proper names for your fields
    // uncomment this line to dump the object to stdout, which should be the web page
    // echo "\n<!-- " . print_r($form,TRUE) . " -->\n";
    $user_fname    = rgpost('input_1_3');
    $user_password = rgpost('input_2');

    // create the SoapClient
    $post_data = array('name' => $user_fname, 'pwd' => user_password);
    $client    = new SoapClient("my.wsdl");
    $the_file  = $client->getFileName($post_data);

    // consider some error handling here
    // find the input ID for your hidden field and change 6 here to the proper field ID
    $_POST["input_6"] = $the_file;
}