Please move _cache directories out of the gallery directories, to make it the plugin FTP friendly.
Imagine I FTP upload some directories, then I FTP-download, and the plugin bloated with _cache folders.
There should be an option available on the Settings page to change the root for the _cache directories.
ps: Lazyest gallery have an option to move _thumbs directory outside the gallery directories.
The plugin is FTP-upload friendly. I have moved the resized images to a subdirectory. Otherwise, you would have all image-150×150.jpg images in your directory.
Moving the _cache directories out of the gallery would break the WordPress media integration.