To clarify a bit more — the GPL allows anyone who receives a copy of the software to sell copies to others for any price they like, or give the software away at no charge. However, regardless of which choice they make, anyone THEY give copies to must ALSO be given the same rights to redistribute copies under the same terms (that is, gratis, or for a fee). These conditions are required/guaranteed by the GNU General Public License that always accompanies the software. As the GPL’s preamble says:
“When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things.”
As a result, the price of GPLed software usually approaches the free-market cost of distributing copies, which is often too small to bother collecting. Since downloads cost only a few pennies for bandwidth, it’s often not worth the bother of trying to sell copies, so the software is usually just distributed at no charge. Nevertheless, being able to charge for the software allows companies to legally sell CD-ROM collections of GPLed Free Software (such as GNU/Linux distributions like Debian), which is very useful service for those without good connections to the ‘net.
This article on the GNU website explains the issue in detail:
https://www.gnu.org/philosophy/selling.html