GPL is about not passing less freedom to your users than you yourself received. That's it. You're free not to use GPL'd software, but the moment you do, it's a

GPL is about not passing less freedom to your users than you yourself received. That's it. You're free not to use GPL'd software, but the moment you do, it's authors are free to impose certain requirements on you, such that you preserve the freedom they gave you for your downstream users.