Combine flour, garlic powder, salt, and pepper in a shallow dish. Roll chicken in flour mixture to coat.

Heat 2 tablespoons oil in a skillet over medium-high heat. Add chicken; cook and stir until no longer pink in the center and the juices run clear, 8 to 10 minutes. Transfer to a plate.

Heat remaining 1 tablespoon oil in the same skillet over medium heat. Add celery, bell peppers, and onion; cook and stir until slightly tender, about 5 minutes.

Meanwhile, whisk ketchup, lemon juice, pineapple with syrup, and brown sugar together in a bowl.

Return chicken to the skillet, pour in pineapple mixture, and bring to a boil. Cook and stir chicken and vegetables in the sauce until heated through, 2 to 3 minutes.