In a large bowl, whisk together all the ingredients for the chicken marinade, then add in the chicken breasts, and toss to coat. Cover and refrigerate for at least 2 hours or overnight.
Using a food processor, shred the cucumber. Then wrap the processed cucumber in a towel or cheesecloth and squeeze out as much water as possible.
In a small bowl mix together the cucumber with the remaining ingredients for the tzatziki sauce. Cover and refrigerate for at least 30 minutes.
Heat oven to 400 degrees. Spray a large baking dish with nonfat cooking spray. Place the chicken in the baking dish and cover with foil. Roast in oven for about 30-35 minutes. Remove foil and return to oven, roasting until chicken is fully cooked through and begins to brown. Remove from oven and let rest for 5 minutes.
Slice chicken into thin strips and divide evenly on top of each pita. Top each with 2 tbsp tzatziki sauce, onions, and tomato slices.
Notes
The entire recipe makes 6 servingsThe serving size is 1 gyro