Is GPL Illegal?
Can a GPL be enforced?
Despite the FUD, as a copyright license the GPL is absolutely solid. That's why I've been able to enforce it dozens of times over nearly ten years, without ever going to court.
Enforcing the GNU GPL - GNU Project - Free Software Foundation
Software under the GPL may be run for all purposes, including commercial purposes and even as a tool for creating proprietary software, such as when using GPL-licensed compilers. Users or companies who distribute GPL-licensed works (e.g. software), may charge a fee for copies or give them free of charge.
GNU General Public License - Wikipedia
GPL is enforceable as it's essentially a copyright license. The copyright holders of the GPL software can choose to enforce the GPL on the distributed or derivative works of the software. For example, the FSF holds the copyrights on many pieces of the GNU system, such as the GNU Compiler Collection.
GPL License - Top 10 Questions Answered - Mend
What happens if you violate the GPL?
When you release your software under the GPL, it means you give anyone a license to use your software under some terms and agreements. If somebody violates the agreement, they are in breach of contract with you. This means you can sue them in a court of law.
What can/should I do when I see a violation to GPL restrictions?