Technically the GPL [does not **prohibit** selling software][1]. But it does **require** that any purchaser must be allowed to distribute the software to anyone else, [free][2]. 

Clearly this requirement effectively prohibits many business models for selling software for a living! You may be able to make some money by selling other things along with the software: support, warranties, training, documentation, consultancy, hardware... 

If you own the copyright on the software, you can dual-license it: sell a paid version under different license terms.


 [1]: http://www.gnu.org/licenses/gpl-faq.html#DoesTheGPLAllowRequireFee
 [2]: http://www.gnu.org/licenses/gpl-faq.html#DoesTheGPLAllowRequireFee
 [3]: http://www.gnu.org/licenses/gpl-faq.html#DoesTheGPLAllowMoney