In a bowl, mix Greek yogurt, minced garlic, lemon juice, oregano, salt, and pepper to create the marinade.

Add chicken breasts to the marinade, ensuring they are well coated. Marinate for at least 1 hour or overnight.

Preheat your grill or skillet over medium-high heat. Cook the marinated chicken for 6-7 minutes on each side until golden brown and the internal temperature reaches 165°F (75°C). Let it rest before slicing.

Grate the cucumber and remove excess moisture. In a bowl, combine the cucumber with remaining Greek yogurt, a bit more garlic, lemon juice, and season with salt.

Warm pita bread on the grill or in a skillet until soft.

Slice the chicken and layer it on the warmed pita along with tzatziki and any optional toppings like lettuce or tomatoes.