How does GPL work?
The GPL requires that the licensee provide complete sources when the work is distributed. If these sources have been modified or added to, then the modification and/or additions are to be contributed back to the community. All works derived from the original GPL work, whether through a combination of sources or use of GPL libraries at compile time, must also be licensed under the GPL license and therefore be freely available. (Note: the LGPL is less restrictive regarding library usage.