As per the Plugin FAQ use,
wp_loginout($_SERVER['REQUEST_URI']);
Sorry that’s entirely incorrect, that function prints out the login/out url which you do not need/want for the purpose of using within a filter, instead use,
wp_redirect( $_SERVER['REQUEST_URI'] );
…to return the user to the page they were on.
If that does not allow you to preserve the form field values during the pre submission filter then you might need to pass the form values into the filter and return them. Surely GF would handle this for you.
Let me know.