user_chroot acts as chroot on Unix/Linux systems.
Permission system of Gallery3 is not adapted when you want to deal only with private albums (one user = one private album). This module permit to restrict an user to only one album (and its sub-albums). It does not overload the permission system, so you can continue to manage permissions in a chroot. It does not make an album private (eg: users not chrooted can access it unless you specificaly deny access)
After installation, go to Admin -> User/Groups. Edit or create an user, and set the "Root Album".