I think you find magnet schools in many of the larger school districts in the country. LA Unified, NYC has magnets are two examples. There are magnet schools in S.F. but I don't know of any that are specifically for "gifted" students. There certainly are many private schools in the Bay Area and S.F. area that call themselves schools for gifted students. Magnet schools need to have a large population to attract students from so they tend to be more available in large cities.