Heat the oil in a large skillet over medium heat. Add the chicken style seitan, and cook until browned, stirring occasionally. Add the bell pepper, onion, and garlic. Cook and stir until tender, but still crisp.
Stir in the pineapple juice, brown sugar, vinegar, soy sauce and cornstarch. Reduce heat to low; cook and stir until thickened. Mix in the pineapple chunks just before serving.