Username auto generation for Gravity Form User Registration Add-on

gform-generate-username.php

<?php
/**
 * Username auto generation for Gravity Form User Registration Add-on
 *
 * see https://www.gravityhelp.com/documentation/article/gform_username/
 */
add_filter( 'gform_username', 'auto_username', 10, 4 );
function auto_username( $username, $feed, $form, $entry ) {

	//$username = strtolower( rgar( $entry, '2.3' ) . rgar( $entry, '2.6' ) );
	$username = sanitize_user( current( explode( '@', $username ) ), true );
	
	if ( empty( $username ) ) {
		return $username;
	}
	
	if ( ! function_exists( 'username_exists' ) ) {
		require_once( ABSPATH . WPINC . '/registration.php' );
	}

	if ( username_exists( $username ) ) {
		$i = 2;
		while ( username_exists( $username . $i ) ) {
			$i++;
		}
		$username = $username . $i;
	};
	
	return $username;
}