If you are doing Calc AB in 9th grade, you would do BC next year, or begin taking college math at the best, closest college to you. You could be halfway through a math major before you even enter college. You should most definitely NOT do a gap year in China. Math skills are highly valued right now, and probably always will be. Your ability to learn and do higher math is best in your young years. You don’t want to waste that year just to be in China. If I were you, I’d be pushing ahead with math, maybe Comp Sci, engineering, hard science, actuarial science, if that’s where your interests lie.
I have an alternative proposal. Ask your parents to send you to China each summer, for summer school in Chinese language and math/science/tech, whatever you like. That way, you would not get off track, you’d continue to zoom ahead in your education, and you’d get your Chinese language (and cultural skills) fluent, without wasting that year after college on a gap year that doesn’t fit your educational goals.