1. In a mixing bowl, combine olive oil, garlic, lemon juice, oregano, salt, and pepper. Add chicken slices and marinate for at least 30 minutes.
2. Heat a skillet over medium-high heat. Cook the marinated chicken until fully cooked, about 5-7 minutes per side.
3. In another bowl, mix Greek yogurt, lemon juice, garlic, dill, salt, and pepper to make the tzatziki sauce.
4. Warm the pita bread in a skillet or oven.
5. Assemble the gyros by placing chicken, tzatziki sauce, tomato, cucumber, and red onion on the pita bread. Serve immediately.