I ran the Application Installer for nGallery. It created the files, directories, and all that but it doesn't seem to want to run. The documentation site you have listed http://www.ngallery.org/docsDefault.aspx does not work. When modified to http://www.ngallery.org/  you go to a strange travel photo site.

So, how does one actually use nGallery? 

Looks like they gave up that domain.