Combine the soy sauce, sugar, garlic powder, and ground black pepper in a bowl. Stir to mix.
Place the ahi tuna in a resealable bag. Pour the soy sauce mixture into the bag. Release the air and seal.Refrigerate and Let the tuna marinate for at least 3 hours.
Remove the tuna from the fridge. Spray high heat cooking oil grill spray on the grill and start to grill the ahi tuna for 4 to 5 minutes per side. Use a spatula to turn the tuna over so that the meat will not get damaged.