Pound out the chicken thighs to flatten them.
Mix the marinade ingredients (garlic, vinegar, lemon juice, yogurt, olive oil, oregano, onion powder, cumin, coriander, paprika, chili flakes, salt, and pepper) and pour over the chicken. Marinate for a few hours or overnight.
Preheat the oven to 400°F.
Stack the marinated chicken on a vertical spit or skewers and roast for 60-90 minutes, basting occasionally.
Make the tzatziki sauce by mixing the diced cucumber, lemon juice, olive oil, yogurt, dill, garlic, salt, and pepper.
Make the Greek vinaigrette by blending the vinegar, lemon juice, olive oil, oregano, and Dijon mustard.
Warm up some flatbreads and assemble the gyros by slicing the roasted chicken and serving it with the tzatziki sauce, Greek salad dressed with the vinaigrette, and the warm flatbreads.