Yes, your GPA will hold you back, without a doubt. A high MCAT is how you'll be able to show that your GPA is not an accurate reflection of your abilities as a student. Do whatever you can to score as highly as possible. Your experience in TFA will help you, but if your MCAT score is low, then it might not be enough.
The schools you apply to will depend on your MCAT score.
As for your PS, you should just write about your personal journey to medicine. Why TFA? Why premed with a math major?