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:
You can freely use the program. You can freely make copies, modify and distribute them. You can download the program and its source code from Gwyddion web pages and modify it as you want.
If you decide to distribute it, the modified code is still covered by the same license. In particular, you have to offer the source code too.
The same holds for extensions, e.g. if you write an import module for a new file type or a new data analysis function it has to be licensed under GNU GPL (when it is distributed – if you use it privately, do whatever you wish with it).
However, it is also possible to execute third-party programs from Gwyddion and these do not necessarily have to be distributed under the same license – if they are not derived works of Gwyddion (which, admittedly, is not always easy to determine).
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.