From the Grapevine informs us that both gyro and shawarma are derived from yet another sandwich, the Turkish Doner Kebab. One main difference with the two lies in their origins. The gyro is actually more Mediterranean than Middle Eastern, as it originates in Greece. The shawarma, on the other hand, is harder to pin down, although Difference Between says it is Arabic and comes from the Levant (an Eastern Mediterranean region of Western Asia). While that is all very interesting, should you be a culinary ethnographer, if you're just a hungry person trying to make up your mind on what to order, you're probably more interested in what makes the gyro taste different from the shawarma and vice-versa.
Both may be served on pita bread, are meat-based, and often are accompanied by fries. Gyros typically are made with lamb, beef, or pork, while shawarmas may be made from lamb, chicken, or turkey. The primary differences, however, lie in the preparation and garnishes. Gyro meat is ground and shaved and cooked with marjoram, oregano, rosemary, and thyme. It's then topped with tomatoes, onions, and tzatziki sauce. Shawarma meat is cut into chunks and seasoned with cardamom, cinnamon, cloves, and turmeric. It may be garnished with fattoush, tabbouleh, tahini, or even pickled mangoes. Does that clear things up for you? Even if you're still a bit confused, no matter. Just order either, or both, and you're bound to be satisfied since they're equally delicious.
