The only web page in GSP is the one that hosts the Gallery user control. In a default installation that means default.aspx. The source code uses lots of user controls that are loaded dynamically depending on the query string. One of these is the login.ascx user control.
Since GSP doesn't include a dedicated aspx page for login, I think what you need to do is create one. You can copy the contents of the existing login control (gs\pages\login.ascx). Then I believe you can use URL rewrite to sense a request to the login page (the query string will look like "default.aspx?g=login&ReturnUrl=XXXXXXX").
Roger Martin
Creator and Lead Developer of Gallery Server Pro