Using this snippet you can change the default register link with your custom registration page link (created by GF form user registration add-on) and also optionally change the "Register" text.

custom_register_link.php

<?php
add_filter( 'register', 'sjaved_register_link' );
function sjaved_register_link( $link ) {

	/*Required: Replace Register_URL with the URL of registeration*/
	$custom_register_link = 'Register_URL';


	/*Optional: You can optionally change the register text e.g. Signup*/
	$register_text = 'Register';

	$link = sprintf('<a href="%1$s">%2$s</a>', $custom_register_link, $register_text) ;	

    return $link;
}