If you take undergrad classes, those will affect your undergrad GPA. If you take graduate classes, they will contribute to your graduate GPA. Postbac students are enrolled as graduate students because they already have a degree, but GPA depends on the level of the course you are taking.
On the AMCAS application, you would label it as Post-Baccalaureate. AMCAS calculates a PB GPA as well. As long as they are undergrad classes...they will contribute to your undergrad GPA.