Actually, GPL isn't "Free of charge." If you read the GPL, it clearly states that it is not necessarily free of charge, but it guarantees you the freedom to modify/use the software released under it however you wish (within the limitations set forth in the license that are needed to allow others the same freedom).
"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
them 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."
http://www.gnu.org/licenses/gpl.htmlSorry, it annoys me when people talk about "Free software" and make it sound like there is no charge, ever.