Legal Question in Criminal Law in California

Only if California secedes from the Union.

Yes. They are illegal throughout the United States, because they broadcast signals in ways forbidden by the FCC. The government aggressively prosecutes people who use these devices, because of their potential to help criminals avoid detection.

My mistake: When I posted my prior answer I thought you asked if these devices were *illegal*, not legal. I should have double-checked what you wrote before I posted my response. The correct answer is no, they are not legal, for the reasons I stated.

The FCC prohibits the use of nearly all types of radio frequency "jamming" devices. It's hard to see why anybody outside a war zone would want to "jam" a GPS device of the type commonly used for navigation. Or are you referring to GPS "tracking" devices used by police agencies, private detectives, and fleet vehicle owners. It would also be illegal to attempt to "jam" these types of devices. If you are worried about your movements being monitored by law enforcement, take the battery out of your cell phone.