In piwigo there is a config option:
$conf['guest_access'] = false;
with this option there will be no photos shown unless you are logged in
(but i think you now this option ;-))
At the moment I create a generic user (user can only see photos; no change of pwd, tags, photos ..) called wp_pwg with a password.
A solution for the problem with the several galleries maybe is solved in the piwigomedia plugin. In this plugin it is possible to pre define several URLS in the admin page of the plugin. In this admin definition it should also be possible to set the user login and password and store it in a config file. (e.g. the password of the database login also saved in the wp-config.php file)
Another topic would be the widget. I have not found a solution to hide the sidebar befor someone logged in. (I did not search yet)
mfG Matthias