Gwyddion is covered by GNU General Public License (GNU GPL). The full license text is also included as file COPYING in the source distribution (MS Windows installers contain it as file COPYING.wri). In brief, this license means that:

The main reasons why the program is covered by this kind of license are here: first of all, this licensing policy enables us to make modular program that can be easily developed by many people from different institutions. Second, this license protects the rights of developers that their code, here given to the public, cannot be copied and used for closed proprietary products.