GravityForms + User Registrations plugin: Use first name and last name in the form to generate username out of both. Append number if the username exists.

gravityforms_username.php

<?php
/**
 * Check if there is still pending registration for specific username
 * @param $username
 *
 * @return bool
 */
function dg_check_pending_registration($username) {
	global $wpdb;
	$table = $wpdb->prefix . 'signups';
	$query = $wpdb->prepare("SELECT COUNT(*) as count FROM {$table} WPS WHERE WPS.user_login='%s'", $username);
	$total = $wpdb->get_var($query);
	return $total > 0;
}

/**
 * Generate username
 *
 * @param $form
 * @param $feed
 * @param $entry
 *
 * @return string
 */
function dg_gf_generate_username($form, $feed, $entry) {
	$username = "";
	// This is taking the field ids directly from User Registration feed (first name and last name)
	$firstname = rgar($entry, $feed['meta']['first_name']);
	$lastname = rgar($entry, $feed['meta']['last_name']);
	if (!empty($firstname) || !empty($lastname)) {
		$username = '';
		if (!empty($firstname)) {
			$username.= $firstname;
		}
		if (!empty($lastname)) {
			$username.= $lastname;
		}
		$username = strtolower($username);
	}
	// Generate random username, append numbers to joe if needed.
	if (username_exists($username) || dg_check_pending_registration($username)) {
		$i = 2;
		while (username_exists($username . $i) || dg_check_pending_registration($username . $i)) {
			$i++;
		}
		$username = $username . $i;
	};
	return $username;
}

/**
 * Generate WordPress username
 * @param $username
 * @param $feed
 * @param $form
 * @param $entry
 *
 * @return string
 */
function dg_gform_username($username, $feed, $form, $entry) {
	if (!function_exists('username_exists')) {
		require_once (ABSPATH . WPINC . '/registration.php');
	}
	$_username = dg_gf_generate_username($form, $feed, $entry);
	if (!empty($_username)) {
		$username = $_username;
	}
	return $username;
}
add_filter('gform_username', 'dg_gform_username', 0, 4);