The MHX2 importer is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
The reason to choose GPL2 rather than GPL3 is to make it compatible with Blender’s license. The MHX2 exporter is released under GPL3, however, to be compatible with MakeHuman.
Extra content added by the MHX2 importer, in particular the various rigs, is released under is released under Creative Commons CC0. You can copy, modify, distribute and perform the work, even for commercial purposes, all without asking permission.
Content imported from MakeHuman maintains its original license, to the extent that that is possible. The MakeHuman Licence.