This depends on where you purchased the shawarma, as you do not know exactly what ingredients are included, or how it was made.
Therefore, pre made Shawarma may be very tasty, but not a healthy option. However, if you make one for yourself at home, then you can choose to use healthier methods and ingredients.
Just keep in mind that the marinated meat can be very high in fat, with a lot of salt and sugar that can be damaging for your health when consumed often.
By making shawarma for yourself, you can reduce the amount of oil used whilst cooking, and less salt if need be.