Place chicken breast, marinara sauce, garlic, salt, and pepper into slow cooker. Cover and cook on high for 4 hours, or on low 8 hours.
Once chicken is done, remove and cut into 1-inch pieces. Add back to slow cooker.
Cook gluten-free penne until al dente. Once done, drain and add to slow cooker.
Add 1 cup mozzarella and 1 cup parmesan over pasta, then stir everything together. Top with remaining 1 cup mozzarella and place the lid back onto the slow cooker (use the warm setting if you have it).