As i remember it’s default iOS behaviour. You can’t directly ask system to show login view, but you can only set handler, that will be called once user authenticated. Login view is shown automatically once that handler is setup, but it shown by system only 3 times (across all games). So the only workaround is to show custom dialog for user to go to settings and to login into GameCenter.
I haven’t worked with GameCenter for a long time, so may be i somewhere wrong, or situation already have changed.