Other Obligations in the GPL

The GPL doesn't grant unconditional licenses. Those who copy and distribute verbatim copies of a Program are required to:

...Conspicuously and appropriately publish on each copy an appropriate copyright notice and disclaimer of warranty; keep intact all the notices that refer to this License and to the absence of any warranty; and give any other recipients of the Program a copy of this License along with the Program. ( GPL section 1.)

Those who create and distribute derivative works are also required to:

a) Cause the modified files to carry prominent notices stating that you changed the files and the date of any change. ( GPL section 2)

c) If the modified program normally reads commands interactively when run, you must cause it, when started running for such interactive use in the most ordinary way, to print or display an announcement including an appropriate copyright notice and a notice that there is no warranty (or else, saying that you provide a warranty) and that users may redistribute the program under these conditions, and telling the user how to view a copy of this License. (Exception: if the Program itself is interactive but does not normally print such an announcement, your work based on the Program is not required to print an announcement.) ( GPL section 2.)

These provisions protect the integrity and reputation of the original authors and ensure that subsequent licensees know that the GPL applies to that software.