Short answer: Until you go to a graduate school, anything you do is part of your undergraduate GPA (Although AACOM only counts the most recent grade if you repeated a course, whereas AAMC treats it as essentially two separate courses).
Longer answer: So let's say you completed your Bio, Physics, Chem, and O-Chem at Roast Beef Tech and graduated with a 3.2....You can then go, even to a community college really, and enroll in Health, Geology, Nutrition, Pharmacology, Astronomy, Engineering or whatever, and you can boost up your science (and overall obviously) GPA. ADCOMS might question you about taking certain courses at the community college level, but at the end of the day, A's are A's, which boost your GPA, get you interviews, and put you in a situation where you even get to explain yourself.