The GPL says explicitly that the act of running the program is "not restricted", i.e. allowed without restrictions. I'm not sure how much clearer you can possibly get in law.
It's true that a few jurisdictions have insane definitions of "copying" that makes even transient copies, such as those exisint in RAM of a program executed from the hard-disc "copies" in the copyright-sense. In those jurisdictions, you require a permission from the copyright-holder to run a program, because running it includes making copies of it.
More sane jurisdictions discount such temporary functional copies from their definition of "copy" and no permission is required.
However: even in those jurisdictions where you -do- need permission to run a program, it is quite clear that the GPL grants you this permission.