Almost any food's calorie content can be found on Fitday. For something like a gyro which is a combination of stuff you may have to add multiple foods (the meat, the pita, sauce, etc) to find the totals.

Luckily for you they do have a general gyro calorie content table on Fitday but you never know how much meat/condiments each place put on their stuff so just use it as a reference point.

Gyro sandwich (pita bread, beef, lamb, onion, condiments), with tomato and spread
Nutrition Facts
Amount Per 1 gyro
Calories 630.09
Calories from Fat 131.14
% Daily Value *
Total Fat 14.57g 22%
Saturated Fat 5.69g 28%
Polyunsaturated Fat 1.59g
Monounsaturated Fat 5.25g
Cholesterol 128.1mg 43%
Sodium 1011.7mg 42%
Potassium 777.77mg 22%
Total Carbohydrate 77.26g 26%
Dietary Fiber 3.88g 16%
Protein 44.8g 90%
Alcohol 0g