Whisk together oil, wine, garlic and oregano in a baking dish. Add the pork and turn to coat. Cover and marinate in the refrigerator for at least 4 hours or overnight.
Preheat grill to high.
Remove pork from marinade and season with salt and pepper. Grill until golden brown on all sides and cooked to an internal temperature of 150 degrees F. Let rest 5 minutes and slice into 1/4-inch thick slices.
Build gyros by drizzling each pita with the yogurt sauce, then adding mesclun, onion, and meat. Drizzle with more sauce and garnish with oregano.
Combine all the ingredients, cover, and let sit in the refrigerator for at least 30 minutes before serving.