Serve with a lightly toasted Grain-free Pita, freshly made Keto Tzatziki and a couple of thin slices of tomato. I would suggest making the Tzatziki and pita the night before. It is also a good idea to prepare and marinate the pork tenderloin overnight to maximize the flavor of the marinade. Note that the meat will become much paler as it marinates; that is because of the vinegar. This is an expected effect.
If you do not want to make the grain-free and sugar-free pitas, you can also eat the pork gyro topped with tzatziki as a main dish and serve it with a Greek salad side dish. Omitting the pitas does lower the total carbohydrates for the meal. However, if you have been mindful of your carbohydrates during the day this meal should fall within daily guidelines for most people. If you are on a very low carb Keto plan, you may be better not to eat the pita, or have half of one. You know your metabolic needs best.