The License section in the project page shows it is under GPL-3.0+:

[quote]
This program 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 3 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. See the GNU General Public License for more details.

http://www.gusnan.se/devilspie2/