Nothing prevents you from paywalling a GPL program as long as you distribute sources to legitimate users on request. Free as in freedom, not as in beer.
"The freedom to run the program as you wish, for any purpose" - "The license must not restrict anyone from making use of the program in a specific field of endeavor."
"The freedom to study how the program works, and change it so it does your computing as you wish" - "The program must include source code (...) The license must allow modifications and derived works"
"The freedom to redistribute copies so you can help your neighbor" - "The program (...) must allow distribution in source code as well as compiled form."
"The freedom to distribute copies of your modified versions to others" - "The license must allow modifications and derived works, and must allow them to be distributed under the same terms as the license of the original software."
That's why almost all software considered "open source" is also "free software" and vice-versa.
"Under pressure from the movie and record companies, software for individuals to use is increasingly designed specifically to restrict them. This malicious feature....is the antithesis in spirit of the freedom that free software aims to provide. And not just in spirit: since the goal of DRM is to trample your freedom, DRM developers try to make it hard, impossible, or even illegal for you to change the software that implements the DRM.
Yet some open source supporters have proposed “open source DRM” software. Their idea is that, by publishing the source code of programs designed to restrict your access to encrypted media and by allowing others to change it, they will produce more powerful and reliable software for restricting users like you. The software would then be delivered to you in devices that do not allow you to change it.
This software might be open source and use the open source development model, but it won't be free software since it won't respect the freedom of the users that actually run it."