On AMCAS, there is a separate column for postbac grades (as there also is for graduate grades). However, when they calculate total undergraduate level cum, there is a combined ug and postbac figure on the application as well. Thus most people assume that is the figure med schools use as your cumulative GPA with respect to cut-offs, giving figures to ranking services etc. Graduate grades are never combined, so don't help your GPA. Osteo is different if there are retaken courses in the postbac -- a course that is retaken will allow you to supplant a prior grade with a subsequent one. Thus a GPA can jump up faster, as you aren't simply averaging everything.