Custom User Registration at wp-login.php?action=register

You can replace the WordPress registration with your form. Create a registration form and configure that Configure the user registration feed. You might have done both of these already.

Then create a page and embed the registration form in that page. If you access that page you created, you will see your registration form there, just like any other form.

Now, configure the settings under Forms > Settings > User Registration.

This is the URL (but on your server):
http://example.com/wp-admin/admin.php?page=gf_settings&addon=User+Registration

Check the box which says "Enable Custom Registration Page". That will ensure any access to wp-login.php will be redirected to your custom registration page.

Screenshot http://minus.com/mbe0RcaBgT

Log out of WordPress, then try accessing your site at wp-login.php and see if you are directed to your custom registration page. If everything is configured correctly, you will be.