Gravity form default and not wp-signup.php

I have purchased the user reg. upgrade for my WP Multisite and have setup a form as my reg. however i can link to it in the menu but anything WP sends anyone to registration ie. to setup another blog or even when it redirects someone to registration after typing in a nonexistent blog ie. http://nonregisteredsub.domain.com, it always utilizes the standard wp-signup.php. How can i bypass the wp-signup.php and program wp to use only my custom registration page? also... I need a text field followed by ".domain.com" (on the same line) as in the wp-signup.php when they are registering a new sub domain. How do I do that?