The Joomla! core team recently publicaly stated that Joomla! 1.0 and 1.5 are released under the pure GPL.  They also confirmed that as derivitive works, extensions must also be released under the GPL.

The RSGallery2 Team applauds the Joomla! core team for taking this stance.  As stated in our FAQ, we love open source software and believe open source benefits everyone involved; developers, companies, clients and end users.  We see this move as leading to an increase in higher quality extensions.

Long Live Open Source!