Combine Flour, Garlic Powder, Salt, And Black Pepper In A Shallow Dish. Roll And Coat Chicken Cubes In Flour Mixture. Heat 2 Tablespoons Vegetable Oil In A Skillet Over Medium-High Heat. 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. Heat 1 Tablespoon Vegetable Oil In The Same Skillet Over Medium Heat. Cook And Stir Celery, Green Peppers, And Onion In Heated Oil Until Slightly Tender, About 5 Minutes. Return Chicken To The Skillet. Whisk Ketchup, Lemon Juice, Pineapple, And Brown Sugar In A Bowl; Pour Into The Skillet; Bring To A Boil. Cook And Stir Chicken And Vegetables In Sauce Until Heated Through, 2 To 3 Minutes.