Read http://www.gnu.org/licenses/gpl-3.0.html and http://www.gnu.org/philosophy/selling.html
Except for one special situation, the GNU General Public License (GNU
GPL) has no requirements about how much you can charge for
distributing a copy of free software. You can charge nothing, a penny,
a dollar, or a billion dollars….The one exception is in the case where binaries are distributed
without the corresponding complete source code. Those who do this are
required by the GNU GPL to provide source code on subsequent request.