From the latest licences.php file we can see that "Joomla! includes or is derivative of works distributed under the licenses listed below."
* BSD License
* GNU Lesser General Public License (GNU LGPL) version 2.1
* MIT License
* PHP License version 3.0
The fact that the PHP Licence itself is not compatible with the GNU GPL Licence itself is a different issue and one for another time.
The Free Software Foundation list numerous licences as being GPL compatible so if it is good enough for Joomla and the Free Software Foundation why is it not good enough for the Joomla Extensions Directory?