A Brief History of the GPL
The GNU General Public License was originated by the Free Software Foundation's Richard Stallman. Stallman conceived the GPL, not surprisingly, after a software dispute involving source code. The first version of the GNU General Public License appeared in 1988, and the GPL has continued to evolve over the years into its current form, which is Version 2, released in 1991. A few other derivative licenses are published by the FSF, such as one geared toward software libraries and another geared toward documentation. However, for the sake of our discussion, the GPL is the license that matters.
The Free Software Foundation still maintains the GPL, and defends it. Columbia University law professor Eben Moglen is the lead counsel for the FSF, and they pursue remedy against people in violation of the license agreement terms. To date, the license has never been challenged in court, largely because there isn't much reason to challenge it. Since it's a license, not a contract, most disputes can be settled by the offender simply stopping distribution of software not in compliance with the license, and perhaps making restitution if the offender has profited from illegal distribution.