Make the meatballs per the recipe link above.
Make the tzatziki per the recipe link above.
In a large bowl, combine the tomatoes, cucumber, parsley, red onion, lemon juice, and salt and pepper to taste. Set aside.
Warm each pita bread in the oven or in the microwave.
Assemble the gyro: lay each pita bread flat, and add some shredded lettuce, 4 meatballs (halved if you prefer), some cucumber and tomato salad, feta cheese, and drizzle with tzatziki sauce. Fold together, and serve. You can wrap it with foil or hold it together with a toothpick to make it easy to serve.