It's been about 10 years since I last used it, but looks like it's still basically the same thing; reads your photos directory, creates html pages with thumbnails and links to bigger pictures based on the directory structure. Can display EXIF data next to the images if you want.
But it's pretty simple. You just have to recursively go over the folder, and get each photo. Make sure there is a thumbnail, and build your HTML.
Some generators (e.g. gostatic) can track changes and dependencies, rebuilding just what changed.
:p
Personally I like and use `lazygal`: http://sousmonlit.zincube.net/~niol/playa/oss/projects/lazyg... Source: https://bitbucket.org/niol/lazygal
It's live and maintained. I also contributed some code fixes and theme to it.