This is a gallery from the old days (2005) when joomla! started.
It had a wide reach in 2008 when it was recommended in several books.
Actaully it has reliable basic functions and runs without hassle in joomla! 3x.
The use of RSGallery2 is free as in free beer. This applies to private and commercial use
The forum response comes within appropiate time and the sources are in github.
Site: Gallery view
Backend: Upload Drag Drop view
Backend: Control Panel
Backend: gallery view
Backend: images view