You need to bear in mind that the GNU General Public License is governed by a body called the Free Software Foundation whose aim (presumably) is that all software should be free. In my view, this undervalues software developers and it devalues the software they create. If I supplied free steering wheels to a car manufacturer, would that give me the right to insist that they sell their cars for free?

Apparently, there's also something called the GNU Lesser General Public License although, to be honest, I couldn't really see what the difference was...