Old RSGallery2 logo
The beginning
Ronald Smit started the creation of rsgallery for mambo around 2001. Therefor is is called rsgallery.
He hints in 2004 that his relationship with rsgallery is three years so it started all in 2001. The oldest copyright inside the actual files is from 2003.
Mambo is the predessor to Joomla! see https://en.wikipedia.org/wiki/Mambo_(software) started in 2000 and published as open source in 2001. After a disagreement with the open source developers it was forked in 2005 and named Joomla with Version 1.0.
For mambo there are only modules or plugins to be found. The german web site http://www.joomlaos.de/ has still some download files from the mambo times. There are plugins and modules but no original download. This leads to the assumption that RSGallery (without a 2 ) could be directly downloaded on his web page http://rsdev.nl
In 2005 Daniël Tulp, Jonah Braun, Ronald Smit, Tomislav Ribicic started the second version of rsgallery2. If it was a fork or in consent with Ronald Smit is not clear. From 2006 he is again in the development with the others.
Surprisingly the changelog tells following story
+ major reorganization of code