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