You shouldn't be passing your password and username as arguments to the via/to methods as these are used for advanced page navigation:
http://www.gebish.org/manual/current/#advanced-page-navigation.
You should add a void login(String username, String password) method to your LoginPage which will perform the necessary actions to log in and then call it after reaching the LoginPage:
at SitePage.
Marcin
P.s. Please ask similar usage questions on the user list (
https://groups.google.com/d/forum/geb-user) in the future. User list is followed by more people and this is a dev list intended for discussion about development of Geb itself.