Gianadda creates a number of subdirectories when it generates a gallery:
_site is the actual generated website. You can browse it locally or upload it to a web server,
_theme contains a copy of the theme files. You usually don't want to change its content, unless
you want to customise the theme on a per-gallery basis. When you change the theme of the gallery with the --change-theme option, the old theme is saved as _theme.backup.
_catalog contains a database with all the information collected from the pictures and folders in the gallery structure (note to hackers: it's an H2 database).