The greatest thing about GNU is that the entire system is what is known as Free Software. Software is "Free Software" when people have the freedom to: Use the software for any purpose, make changes to the software, share the software with others, and distribute modified versions of the software.
Software that isn't Free Software is called proprietary software. It is so-called because a person claims the software to be their property and bans others from sharing and making changes to it.
From an ethical standpoint, writing Free Software is a much more social way to act. Free Software empowers it's users by allowing them to help themselves by making changes they want to the software (or getting someone else to make these changes). It allows people to help their neighbours by sharing the software, proprietary software does the opposite: it makes sharing illegal telling people that it is a criminal offenses to say "yes" when someone asks for help. And Free Software allows people to help their community by distributing improved versions of the software.
Free Software also doesn't descriminate against poorer people or people from developing nations. By allowing them all the above freedoms it permits them to use computers without having to pay impossible amounts of money for the "privilage".
Finally there are the technical benefits. Free Software is free from marketing plots. It doesn't restrict itself to force users to buy extra pieces of software. Each piece of GNU is designed to be as useful as possible. As a programmer you can use the same C programming software that is used in major projects.
Non-Free programs are generally distributed in a machine readable form only. This means that the user cannot see what is going on inside a program. In contrast, Free Software is required to come with source code in a human readable format. As a programmer you can read the source code for any piece os Free Software you like. If there are errors in a program, you can fix them.
This freedom to fix errors and add functionality is what has made GNU software so good. All code is available for peer review.
Free Software will change the world for the better