The GPL does not say you can't charge for software. You can charge. Someone else can take your source code and give it away for free. All is fair.
Titan is the only moon that is known to have an atmosphere of significant composition, which in this case is almost entirely nitrogen. The cloud is about half the size of the United States, and the images are astounding."NASA's Cassini spacecraft captured the image of the 1,490-mile-diameter (2,400-kilometer-diameter) cloud on December 29, 2006. The cloud's presence fits predictions that Titan has a "methane cycle" similar to Earth's water cycle, with bodies of liquid methane evaporating and forming clouds that rain material back down on the surface.
Here is the goverment page regarding the fee and its non-taxable status: