Set username by using sanitized first and last name.

gf-multisite-username-fix.php

<?php
/**
 * Plugin Name: GravityForms Multisite Username Fix
 * @package     GravityFormsMultisiteUsernameFix
 * @author      Bernhard Kau
 * @license     GPLv3
 *
 * @wordpress-plugin
 * Plugin Name: GravityForms Multisite Username Fix
 * Plugin URI: https://kau-boys.de
 * Description: Use first and last name for a sanitized and unique username.
 * Version: 0.1
 * Author: Bernhard Kau
 * Author URI: https://kau-boys.de
 * License: GPLv3
 * License URI: http://www.gnu.org/licenses/gpl-3.0.txt
 */

/**
 * Set username by using sanitized first and last name.
 *
 * @param string $username The username of the being created
 * @param array  $feed     The Feed object
 * @param array  $form     The Form object
 * @param array  $entry    The Entry object
 */
function gf_ms_uf_set_username( $username, $feed, $form, $entry ) {
        // Get first and last name from the form submission.
        $first_name = rgar( $entry, rgars( $feed, 'meta/first_name' ) );
        $last_name  = rgar( $entry, rgars( $feed, 'meta/last_name' ) );

        // Generate a new username making it compatible with multisite constraints.
        $new_username = gf_ms_uf_get_unique_username( sanitize_user( $first_name . $last_name ) );

        if ( ! empty ( $new_username ) ) {
                $username = $new_username;
        }

        return $username;
}

add_filter( 'gform_username', 'gf_ms_uf_set_username', 10, 4 );

function gf_ms_uf_get_unique_username( $username ) {
        $number            = 2;
        $original_username = $username;

        while ( username_exists( $username ) || gf_ms_uf_is_user_signed_up( $username ) ) {
                $username = $original_username . $number ++;
        }

        return $username;
}

function gf_ms_uf_is_user_signed_up( $username ) {
        global $wpdb;

        return $wpdb->get_row( $wpdb->prepare( "SELECT * FROM $wpdb->signups WHERE user_login = %s", $username ) );
}