1. In a mixing bowl, combine olive oil, minced garlic, lemon juice, oregano, salt, and black pepper to make the marinade.
2. Add the thinly sliced chicken breast to the marinade and mix well. Let it marinate for at least 30 minutes.
3. Preheat a grill pan over medium-high heat. Grill the marinated chicken slices for about 5-7 minutes on each side until fully cooked.
4. Warm the pita bread on the grill for about 1-2 minutes on each side.
5. Assemble the gyros by placing grilled chicken slices on the pita bread. Top with tzatziki sauce, shredded lettuce, diced tomatoes, and thinly sliced red onion.