This is the cool and creamy sauce that usually is served as a condiment on Greek Gyro sandwiches. The recipe calls for the thicker, Greek Yogurt, also called, "Greek Cheese". If you can't find this type of yogurt, you can use plain, regular yogurt from the grocery store. But in order to get the sauce to the desired thicker consistency, you need to suspend the yogurt in a cheesecloth for a couple of hours to drain some of the water off.
If you don't mind it moderately runny, you can just make it as is and it is still delicious, just a little more messy.
Make this with Recipezaar recipe #285900, Gyros- An Authentic Recipe For Making Them At Home