Most foods are customized a bit to match local tastes - Its highly likely that you would probably get to eat the genuine shawarma only in Mid-Eastern or some Mediterranean countries. In US, even Indian food is slightly modified, reducing spiciness or replacing some not so common ingredient (sometimes even some which are expensive or even banned there) . Same is likely to be for Shawarma - I have eaten in India & in US , I doubt if any of them would really be 100% as how it is made in the home country(ies) of the shawarma