Marinate the chicken (quick method): In a bowl, mix sliced chicken with olive oil, lemon juice, yogurt, garlic, and all spices. Let sit while the oven preheats.
Preheat oven to 425°F.
Prepare sheet pans: Sheet Pan 1 — spread marinated chicken, sliced peppers, and onions across a large lined sheet pan. Don’t overcrowd, use two pans if needed. Sheet Pan 2 — spread frozen fries on a second sheet pan. Lightly spray or drizzle with olive oil.
Roast everything: Roast both pans at 425°F for 30–35 minutes. Toss the chicken/veggies halfway through for even browning. Flip fries if needed to crisp. Optional: broil the chicken for the last 2–3 minutes for extra char.
Make the tzatziki while it bakes: Mix all tzatziki ingredients together in a bowl and chill until serving.
Assemble gyros: Warm pita or flatbreads (I like to wrap stacks of 4-5 in a wet paper towel and microwave for 30 seconds). Load them up with roasted chicken, peppers/onions, crispy fries, a big dollop of tzatziki, and any extra toppings you love.