Make the Gyros marinade: just add all the ingredients into a bowl and mix properly. The marinade has to be salty, so give it a taste and add more salt if needed.
Place the meat into the bowl and use your hands to coat all the pieces with the marinade. Let chill in the fridge for at least 30 minutes.
While the meat marinates, prepare the Tzatziki sauce. Peel and grate a large cucumber. Using a paper of kitchen towel, squeeze out as much water as possible from the grated cucumber.
Add the rest of the listed Tzatziki ingredients and mix everything together properly. Store in the fridge, Tzatziki is the best when served cold.
Fire up the grill and start grilling the meat. Flip from time to time, until the meat is nicely browned with a bit of char from both sides. This will take 20-30 minutes, based on how hot your grill is.
Make the salad, while the meat is sitting on the grill. Slice the lettuce, onion and cherry tomatoes. Add olive oil, salt, pepper and mix properly.
Once the meat is ready, we are ready to assemble our chicken gyros. Heat up a piece of flat bread or a tortilla on the grill.
Place the flatbread over aluminum foil, to make the process easier. Start with the salad, then the meat and finish it up with a solid amount of Tzatziki sauce. Wrap the bread up and fix the bottom with the foil.
We are done, our chicken gyros is ready to be served.