GNU is an operating system that offers a set of free open source programs licensed under UNIX. It has many applications and tools that are also provided with the operating system. This allows users online to share all kinds of files with the intention of upgrading and changing the programs by other networked software developers.
However, being an open source program does not mean that the program is free of charge but is free to modify and publish again. You can hence find more documentation on this subject on the official link http://www.gnu.org/