Indeed, this is stated quite explicitly in the GPL itself: > ...nothing other than this License grants you permission to propagate or modify any covered work.

Indeed, this is stated quite explicitly in the GPL itself: > ...nothing other than this License grants you permission to propagate or modify any covered work. These actions infringe copyright if you do not accept this License. Therefore, by modifying or propagating a covered work, you indicate your acceptance of this License to do so.