To use an alternative to the default registration page offered by the s2member plug-in, they recommend the following:
TIP: When creating custom registration forms, if those custom registration forms are also being processed by your own custom routines, be sure that your custom registration routines always make a call to this function: wp_create_user(), or wpmu_create_user() on a Multisite Network. This way the right Hooks/Filters are processed internally by s2Member. If that is not possible, then make sure your custom routine is at least firing this Hook after it creates an account: do_action("user_register", $user_id);
Can someone confirm if the new GF REgistration plug-in uses these calls?