Step 1: Combine flour, garlic powder, salt, and black pepper in a shallow dish.
Step 2: Roll and coat chicken cubes in flour mixture.
Step 4: Heat 2 tablespoons vegetable oil in a skillet over medium-high heat.
Step 6: Cook and stir chicken in hot oil until not longer pink in the center and juices run clear, 8 to 10 minutes; remove and set aside.
Step 8: Heat 1 tablespoon vegetable oil in the same skillet over medium heat.
Step 10: Cook and stir celery, green peppers, and onion in heated oil until slightly tender, about 5 minutes.
Step 12: Return chicken to the skillet.
Step 14: Whisk ketchup, lemon juice, pineapple, and brown sugar in a bowl; pour into the skillet; bring to a boil.
Step 16: Cook and stir chicken and vegetables in sauce until heated through, 2 to 3 minutes.