1. Spread the chicken on a cutting board. With a knife, butterfly the chicken to open and flatten it. Place plastic wrap over it and pound it. This way, it will fit evenly into the loaf pan and the chicken pieces will bind better during baking.
2. In a bowl, mix all the marinade ingredients. Add the chicken and mix well. Cover and refrigerate, preferably for at least 3 hours (or overnight).
3. Line a rectangular loaf pan (base and sides) with non-stick parchment paper and arrange the chicken inside in layers, pressing it down continuously. Once all the chicken pieces are in, press them down firmly with your hands or a heavy glass to make it compact.
4. Preheat the oven to 200°C (fan-assisted). Bake uncovered for 1 hour. Remove any liquid from the loaf pan and let the gyros 'rest' for 10 minutes. Unmold and slice vertically into thin pieces, just like classic gyros.
5. Serve in a warmed Cypriot pita. Spread tzatziki inside, add plenty of chicken gyros, red onions, cucumber, and tomato.
