The GPSTk is open source software and is distributed under the terms of theGNU Lesser General Public License. The LGPL governs the distribution of the software, not the use.
If you have any concerns about what you can and can't do with a program that is licensed under the LGPL, you should visit the FSF's GPL FAQ.
Of particular relevance to results or data which are generated by the GPSTk is the following section:
http://www.fsf.org/licensing/licenses/gpl-faq.html#GPLOutput and