Check this list. http://www.hoagiesgifted.org/schools.htm then search through for the word magnet. Maybe a dozen schools, in 4 states, but most of them in LA. The next question is whether selection is based on achievement test or IQ test. Would you move to LA just for a shot at it. This is not on the hoagies list but interesting http://www.fremont.k12.ca.us/Page/237

An interesting related question is whether there are gifted charter schools. A few states offer some of those. Charter laws vary from state to state. I am not sure "gifted charter" is possible in California. At least, I don't see one on the hoagies list. But in some states charter also means you don't have to live in that exact district. So that's magnet-like.