Make sure field value begins with http://

<?php
// http://www.gravityhelp.com/forums/topic/custom-input-mask-for-a-valid-url#post-157749
// change the 338 to your form ID
add_filter('gform_pre_submission_filter_338', 'force_http');
function force_http($form) {

        // change the 3 to your field ID where the URL will be entered
        // change it a few lines further down as well
        // trim the whitespace first, then make it lowercase
        $haystack = strtolower(trim($_POST['input_3']));

        // check for http:// at the beginning of the string
        $needle = 'http://';

        // if the http:// is not at the beginning of the string, add it and modify the POST value
        if (substr($haystack, 0, 7) == $needle) {
                return $form;
        }
        else {
                $_POST['input_3'] = $needle . $haystack;
                return $form;
        }
}