Make the Gyro Meat: In a medium bowl, combine the ground lamb, salt, pepper and oregano leaves with your hands, mixing until all of the seasonings have been evenly distributed. Cover and refrigerate for at least 1 hour, or overnight.
Preheat oven to 300 degrees F. Place the lamb mixture in the bowl of a food processor and add the onion, garlic and bacon. Process until a smooth puree is formed, 30 seconds to 1 minute, scraping down the sides of the bowl as necessary.
Line a rimmed baking sheet with aluminum foil. With moistened hands, shape the lamb mixture into a rectangle about 8 inches long and 5 inches wide. Bake until the center of the loaf reaches 155 degrees F on an instant-read thermometer, 30 to 35 minutes. Remove from the oven and allow to rest for 15 minutes.
Adjust the oven rack to the highest position (1½ to 2 inches below broiler element) and preheat broiler. Slice the loaf of lamb meat crosswise into very thin pieces (they should be about ⅛-inch thick; no more than ¼-inch thick). Lay the strips on a rimmed baking sheet lined with aluminum foil and broil until edges are browned and crispy, 2 to 4 minutes. (I did this in two batches, as I couldn't fit all of the sliced meat on one baking sheet.) Be sure to keep an eye on it, as the broiler works quickly!
Serve the Gyros: Warm up the bread either in the microwave (30 seconds), on the stovetop or in the oven. Top each with ¼ cup of tzatziki sauce, chopped tomatoes, sliced onions and gyro meat. Wrap with foil and serve.