Consider a voting computer. The software running this computer is licensed under the GNU GPL v3.
Does this license mean that the voters using this computer to cast their votes must be able to modify the code of the software running the computer?
It is true that anyone can modify and redistribute the code of a GNU GPL v3 project. But no one is under any obligation to actually run a modified version that they do not want to run. So yes, you could create your own fork of the voting software and rig it, but no one will run that version on an actual voting computer.
There is a legitimate concern here though, namely that someone will add a feature or fix a bug and with that modification, sneak in some sort of exploit. If this modification makes it through, the person who made the modification could then use it to their benefit. This is one reason why all user contributions should always be thoroughly scrutinized before they are added to a project.
No. Companies distributing devices that include software under GPLv3 are at most required to provide the source and Installation Information for the software to people who possess a copy of the object code. The voter who uses a voting machine (like any other kiosk) doesn't get possession of it, not even temporarily, so the voter also does not get possession of the binary software in it.