The GNU General Public License (GPL)
|“||authorize[s] not only copying but also the creation of derivative works — and the license prohibits charging for the derivative work. People may make and distribute derivative works if and only if they come under the same license terms as the original work. Thus the GPL propagates from user to user and revision to revision: neither the original author, nor any creator of a revised or improved version, may charge for the software or allow any successor to charge. Copyright law, usually the basis of limiting reproduction in order to collect a fee, ensures that open-source software remains free: any attempt to sell a derivative work will violate the copyright laws, even if the improver has not accepted the GPL.||”|
"The GNU General Public Licence (GPL) requires that altered or extra code added to GPL software is also licensed under the GPL. This ensures the propagation of OSS but can cause licensing conflicts if GPL and proprietary software are combined."