1. In a mixing bowl, combine olive oil, garlic, lemon juice, oregano, salt, and black pepper.
2. Add chicken cubes to the bowl and mix well to coat. Let it marinate for at least 15 minutes.
3. Preheat the grill to medium-high heat.
4. Thread the marinated chicken onto skewers.
5. Grill the chicken skewers for about 10 minutes, turning occasionally, until fully cooked.
6. Warm the pita bread on the grill for about 1 minute on each side.
7. Assemble the gyros by placing grilled chicken, tzatziki sauce, tomatoes, red onion, and lettuce on each pita bread.
8. Roll up the pita bread and serve immediately.