GNU is a General Public License (GPL). This comes with some terms and conditions for copying, modifying, and distributing the software licensed under its name. GNU ensures that any software source code licensed under it has to make originating source code open and freely available to all its users. However, freely here means that it is freely available to users for distributing and modifying the code, but they can't impose any restrictions on further distribution, the source code has to be made available.
