Contrary to popular belief I don't just make this stuff up. GPL section 2b says:
"You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License."
And section 11 says:
"BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR THE PROGRAM,..."
Elsewhere it says you can charge a nominal fee for "physically performing source distribution", and it doesn't disallow you for charging for support, but that's it.
See also the faq:
http://www.gnu.org/licenses/gpl-faq.html#TOCDoesTheGPLAllowRequireFee
"The GPL is a free software license, and therefore it permits people to use and even redistribute the software without being required to pay anyone a fee for doing so."
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