In a medium bowl, combine the tuna, pickle, celery, mayo, onion, garlic powder, salt and pepper until well mixed.

Divide the tuna salad between the two pitas, putting the tuna salad only on half of the pita. Put your tomatoes on top, then sprinkle with shredded cheese. Leave the pita open-faced and place in a toaster oven or regular oven. Toast for 4-5 minutes or until the cheese is melty on top to your liking. Save leftover tuna salad in the fridge for up to 3 days.