Should take a gap year after 12th grade?

In general, what's your overall goal for a gap year? Are you hoping to make yourself a stronger candidate for universities you want to apply to? I have some advice but it depends on your goals, and I can't gauge them exactly from what you've said right now.

First, because this one's pretty simple, remember that you can only take the SAT reasoning exam and the Subject Tests a certain number of times before schools will start to look negatively at your application. In general, you should not take the SAT more than three times as a high school student; taking it more than three times will usually be viewed pretty negatively by a college, because they see it as someone just trying to take as many attempts as they can to randomly get a good score. The same goes for the Subject Tests, and with those I'd be even more restrained (try not to take any of them more than twice). So I'm not sure how much a gap year would ultimately help you with that.

As for your extracurricular activities, public speaking and debate, etc.—that kind of thing is much harder to do outside of an actual gap year program, and would be much, much easier for you to do as a freshman in college, because you'd be around lots of other students interested in the same things. The only thing that you would be able to do better through a gap year is definitely to work, and if your goal is to work and save money for college, that's a good reason to take that year off.

So again, it depends on your goals. If you want to save money, then it might be a good idea. But if your overall goal is self-improvement... well, that's kind of the point of college itself, isn't it? So unless you have a strong idea of what you might want to do and participate in, I don't know how much it would really help you in the end.

As for becoming a cancer researcher, I can try to outline what the path for that would look like. That's a very academic career path, as in you'll need a lot of school to do it, and there isn't much you can do outside of school to really set yourself up for it. First, it will take probably 4 years to get a Bachelor's degree (ideally in biology, chemistry, or another life science-related field), and then 5-6 years BEYOND that to get a PhD and/or an MD (medical researchers come from both pathways). A PhD is an academic research degree, while an MD is the degree that physicians and doctors get, but both would enable you to become a medical researcher. To set yourself up to get a PhD or MD, you'd want to participate in research opportunities as an undergraduate, which is something you really can't do during a gap year—those opportunities are really only available to people as undergraduates in college. Then you would need to apply for PhD programs or for medical school after graduating with your Bachelor's degree, through a process fairly similar to this (incidentally, many people take a gap year or multiple gap years between undergrad and those programs as well.)

So, really, I might lean towards telling you to skip the gap year and go to college directly, as that might better suit what seem to be your overall goals. But I don't have all the information I would need to say for sure.