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 hold for extensions (e.g. if you write a module). The only exception are plug-ins that are run as a separate programs. Plug-ins therefore can have any license.
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 my many persons from different institutions. Second, this license protects the rights of developers that their code, here given to public, cannot be copied and used for closed proprietary products.