Don't think HS gpa necessarily will say a lot about your chances to enter med school, let alone choice of speciality. It WILL of course play a role in college admissions, but also there you'll notice a lot of other parameters, like test scores, extracurricular activities etc.
Moving on, a very low gpa in college (or med school) might make progress more difficult, while a high gpa wouldn't necessarily make it considerably easier.
To the OP: Why not use your grades as a guide? Do you easily get good grades in science subjects, or do you work hard to stay above water? Remember, that medicine is initially quite heavy on science subjects, so it's not something you'd want to struggle unnecessarily with. Also, remember never to limit your options too soon. While medicine in itself is a huge and varied field, there's so many more other options out there, that you should consider before embarking on a VERY long journey (remember, that there's aprox. 12 years from you graduate from high school until you will get a real - no-training - job as a doctor).